Need help

I have a 74 Dart with a 318. It's a 3spd automatic 904 transmission. It goes in reverse. It goes in low 1 and 2. It even runs in drive. But while in drive it shifts normally from first to second gear. But it won't shift past second. It just won't go into 3rd. My brother had the car previously. He said he put it away in the winter and it was fine. He said the next spring he pulled it out and had this problem. He told me he had the tranny flushed backwards. I guess he thought something may have been clogged. But it didn't help. I drove it for myself today (first time fast enough for it to shift gears) and it just won't go into 3rd. Does anyone have an idea of what could be wrong?