Mississippi maps are wonderful turtles. And we all loved red eared sliders : )
I do have to say in answer to your quwestion:
No.
A 75 gallon is not enough space for three maps and certainly not for a map and two red ears. If the red ear turns out to be female its not even enough room for her by herself.
You should really consider not using goldfish. There are many health risks to your animals with them from high levels of fat to parasites to chemicals in their bodies absorbed from water tratments on the farms in which they are raised. All can harm your turtles.
Try more insects and cooked fish. Be sure to add some WATER PLANTS soon as the adult RES has a diet that is largely plant based. Too much animal food will result in many problems and early death. You may try anacharis, water lettuce, water hyacinth, hornwort, moneywort and also fruits like canteloupe, honeydew melon, apple,and pear. Some RES will eat squash and carrot as well as green leafy plants like dandelion greens, collard greens and escarole.
Try to aim for slow steady growth rather than rapid growth.