David -
If your pond is 3 feet deep it is will be approximately 1170 gallons. More important than the exact amount of gallons is the space that you have.
A ten foot by 5 foot by 3 feet enclosure is certainly adequate for more than two sliders. Even two female sliders.
If figure even as much as 125 gallons per FEMALE slider. That would only encompass 250 gallons for the two animals that you want. I would add a few more animals with confidence if that is what you want to do. Six would be a nice number of animals and still give you room to spare without stuffing the pond to maximum capacity. Some things to consider with this pond before you get that many animals:
1. Will they be staying out year round?
2. If you get six make sure that the ratio of females to males is larger like no more than 2 boys to 4 girls.
3. Make sure you have a border around the pond so the turtles cannot escape.
4. If you have a wall that prohibits the turtles from physically climbing out of the pond on earth be sure to furnish the animals with suitable basking areas (logs or rock platforms) inside the pond.
5. Please adopt sliders from a rescue organization before you spend your money on them. I work with several of these organizations and can tell you that we get some spectacular animals in. If the pond you have checks out and proves to be a safe turtle friendly one (and we are always willing to give pointers on how to make that way -- should you be new to all of this) you would be able to adopt from most organizations for nothing more than the cost of shipping them to you if they are not a local rescue.
Just some things to consider and good luck with you pond.
Steve