98 s-10 blazer will not shift into third gear shift into 1 st and 2nd fine
have changed filter and fluid

 

ive heard of this as being a common problem after someone changes there filter and fluid because the 3rd shift solenoid is towards the back of the pan and when people go to remove the pan if you hit that solenoid just right you can break it (they can be kinda touchy). did it quit working after the change??

 

Did the condition exist before you changed the fluid & filter? If so, you have likely lost the 3-4 clutch pack and need to rebuild the transmission. If shift solenoid B were to fail or have damaged wiring, it would affect 2nd & 3rd gears.