Anyone who is flushing more than once, needs a newer toilet, it took a while for the manufacturers to catch up with the laws.
You must be doing something to your 60 years old toilet to make it a 3 gallon flusher, because those used to be 5.5 to 7 gallon flushers.
The 3.5 toilets from the 1980s were pretty bad also.
Once the states started passing laws regulating toilets the manufacturers asked the feds to make national standards.
I don’t know when federal involvement started but in the sixties toilets had already gone to 5.5 gallons from 7 gallons and that was followed by 3.5 gallons in the 80s, then the 1.6 in 1994.
The repeated flushes might be more than just the toilet that is the cause. Old plumbing system (like the one in my house) are not designed to handle taking a trud from the toilet to the septic tank with just 1.6 gallons.
our old 1.6 flush toilet would normally block the pipe to the septic because the small amount of water would leave things behind.
With the new (old) 5 gallon toilet there is again enough water to carry it all down.
Sorry, the ‘60s-’80s toilets are 3.0 gal, not 3.5 (not sure if there are different regulations then for different places, but that is the parents’ volume). Thus is it less to flush once old than to flush 2x in the modern requirement.