Is a 90 gallon tank enough for 3 Red eared Slider Turtles?

i would say no. it might be big enough for babies but definitely not for adults

Good lord, no. It wouldn't be large enough for just the two adults. RES are territorial and aggressive. The bare minimum I would recommend is 15 gallons per inch of shell, cumulative. Say, for example, that the adults are 6" each and the juvenile, 3". That's 15 inches total and the minimum you would want would be a 225-gallon tank. Got a spare bathtub? You might get away with using a large wading pool, though.

It all depends on how big your sliders are and what gender. It's 10/15 gallons per inch length of shell.

With multiple turtles in a tank, lets just say you have two 9" adults and one 4" juvenile. While it's not a good idea to house smaller turtles with larger ones. You'll need at least a 155 gallon long aquarium. It's 10/15 gallons per inch length of shell for the largest turtle, and 5 gallons per inch length for every additional turtle.