727 no 3rd wont shift

i did a little reserch and it makes sense that what bob shingler is saying, i have a floor shifter annd i start off in drive in i have first,wont shift i manually put it in second no problems there,and i never tried to put it into 1st manually at 40 mph(for obvious reasons)but i cant help but wonder if i did it would have shifted into 3rd,damn iwish i had of tried that.

One question,on a reverse manual vb what happens when u try to start off in low gear position?
Mine wanted to move very little and i could rev motor freely.

If this is what the problem is the whole time,i will feel like a very green idiot!