1997 Chevy Suburban 1997 Suburban will not shift into 3rd o

I have a 1997 Suburban with 205,000 miles. The transmission shift well into 1st and 2nd gears but as soon as it tries to shift into 3rd it slips out of gear and the engine RPM's ramp up. If I leave the gear selector in 2dn the transmission work great, but if I try to manually shift into 3rd it does the same thing. The transmission has been going through fluid since I got it about 5,00 miles ago. What do I need to do to the transmission to fix this problem?