Hmmmm - sounds more to me (assuming all three are same brand and model and age) more like a small pipe size, partially blocked pipes due to soap scum and grease buildup in them, or blocked vent issue. The latter easy to check - if when the toilets flush they drain completely out and gurgle and make bubbling noises until they refill with the trickle flow from the tank, could be a vent issue. Ditto if flushing one causes the water level in one or more of the others to drop noticeably.
If mineral buildup is blocking the flow through the toilets, I would expect to see a heavy buildup (heavy scale) visible in the outlet from the bowls - if you see fairly clean porcelain in the outlet passage or just minor algae growth in thereif not brushed frequently.
As for the buildup in the toilet syphon (the water passage through the toilet base to the drain lines) thing - only way to check that out would be to dismount one and visually inspect it with a mirror from both ends, or run a sewer camera or fiber optic scope (after removing the trap water for fiber optic scope, which you could rent) through there to visually see. But even in several areas where I have lived/worked with hardness exceeding 100 grains (up to almost 1000 in one location) I have not seen toilets significantly block off in the flow path - a lot of bowl buildup and a thin coating in the syphon, but with a reasonably frequently used and cleaned toilet I doubt mineral buildup is causing it.
Another possibility - when these were installed (or reinstalled after new flooring say, or new toilets put in) maybe they used the rubber sleeve type boots instead of wax seals at the toilet/DWV pipe flange interface - many of those significantly restrict the diameter at that point so, while flowing smoothly, cause the water to drain from the toilet slower. Oops - I see you said 41 year old toilet - so guess that is out unless they have bveen removed and replaced like during a remodel or reflooring or such.
Unless you want to pay about $200-300 minimum (more with fancier toilet) to change one out initially and see if the toilet is the problem, and especially if your lines have not been cleaned in ages (should be done every 5-15 years depending on amount of food waste and especially grease dumped down there), I would personally start with a Sewer and Drain cleaning company. Having them run a sewer camera through each toilet to see what the inside looks like - then if not heavily built up or clogged in there (which if present could be cleared out with muriatic acid but needs dismounting the toilet to do that without hurting the pipes), run a small-diameter jetter through each toilet and down through the drain pipes to whatever point he can then reasonably get a full-size router or jetter in (which might end up being at one of the toilets if no convenient in-house cleanouts - preferably most upstream one if removing a toilet), clear the interior pipes from there to outside the house, then go in at the outside cleanout and continue the cleaning all the way to the street.
One other possible cause if this is a relatively rapid buildup issue (not something developing over a long period of time), if all these are on ground floor and there no basement drains (if you have a basement), is an overfull septic tank if you are on septic. If slow buildup, ditto on no lower-level drains that are not backing up and overflowing, could be leach field (if you have one) slowly choking off and needs refurbishing. Septic tanks typically need pumping every 1-5 years (sometimes longer with large tank and only 1-2 people in household) with the interval depending on tank size, ground temperature and amount of hot water put down the drains, and number of people and amount of food garbage or grease put down the drains.
A full septic tank or slowly-draining leach field can restrict flow into the septic tank, without causing actual backups into the house as long as the leach field is taking the daily flow amount of water on the average (but backs up some and drains slowly during high flow discharging) and as long as the backup during the slow drainage does not get all the way back to a drain.
Note if there is a lower elevation drain which could back up but is not, then septic system would NOT be the cause, because if they were restricting the flow and causing backup in the pipes enough to affect the toilets, that lower elevation drain would be overflowing.
If septic backup is the case - or a partial blockage or general buildup of scum in the pipes, then when flushing (when no other water has been used recently) the flow from the toilet should be full-flow initially for at least several seconds or more as it fills the open part of the pipes, then slowing down as the pipes fill up and start flowing under pressure - sometimes with air gurgling up from the filling pipes into the toilet or other drain traps if the vent is not working right or not plumbed properly at the fixtures.
If the blockage is in the toilet or immediately below it (like at the wax ring seal or maybe at a corroded toilet flange) then the flow should slow down from full flow to a steady but restricted outflow pretty quick - in a second or two.
One other possible cause depending on how your venting is run - if your sewer vent pipe (which comes out at the roof) is blocked with accumulated solids (plumber or sewer cleaner can clean that from the roof or you can do it with a hose while someone runs around inside to be sure no drain is overflowing inside due to a pipe blockage) then that can cause slow drainage of larger volumes of discharged water like from washing machine, toilet, full tub draining - though that is almost always accompanied by the water level dropping down at the bowl outlet at the end of a flush to the point that it does the chugging and gurgling thing as it pulls air from the bowl, before the bowl refills from the trickle refill flow into the bowl from the tank through the under-rim holes.
Lot of previous questions with answers on cleaning sewer lines and clearing partial clogs can be found in the Home > Plumbing and Home > Sewer links under Browse Projects, at lower left.