Three speed transmition

I have a 1974 Ford E-100 (Econoline van) with a three speed transmition. At first it wouldn't move at all when I let out the clutch, then when I tried putting it in first gear, it went in reverse perfectly. It also would go in reverse when I put it back in reverse, but I couldn't get it to go forward at all. I also couldn't get the gear shifter to go into second or third positions at all... What could this be, is it anything I could fix myself (clutch? transmition?....)
Thanks

This reminds me of an old VW Rabbit field car I had.
All I had was 3rd gear, regardless of the shifter position.
Chances are your transmission and clutch are fine. Your shifter linkage is bad.
My guess is that your shifter is on the steering column. There is an elaborate linkage with several plastic and rubber bushings. These bushings are either worn out or part of the linkage has fallen off, because a cotter pin or retaining clip has come off.
Chances are you can inspect the linkage and figure out what has gone wrong.
Often, the floor shift trucks have a similar linkage.
I would suggest start underneath, at the transmission. Have a friend work the shifter from gear to gear, (engine off) and look underneath. You may be able to figure out what has fallen off.
You can get kits with all the bushings and clips to completely rebuild the shifter.

thanks Magicrat - you were right. The linkage to the transmission had come apart. A simple pin and clip held two rods together and the clip had fallen off. the pin was still there, for a simple $1.24, I bought a set of push nuts/clips and it was fixed. Thanks a lot!!!