I'd return them if the lfs gave you the impression that you could keep them in a 75 for 3 years or even or year. Get something that'll be easy to care for in a 75.
Consider the size and number of water changes that will be needed for a fish the size of just one adult 12" oscar in a 75. I have one and 75% weekly water changes are needed to keep nitrate under control.
Smaller fish like JDs or GTs, firemouths, are better for your first tank larger than 29 gal. It just depends on how much work you're willing to put into the tank.
Consider the size and number of water changes that will be needed for a fish the size of just one adult 12" oscar in a 75. I have one and 75% weekly water changes are needed to keep nitrate under control.
Smaller fish like JDs or GTs, firemouths, are better for your first tank larger than 29 gal. It just depends on how much work you're willing to put into the tank.