[Jac Mac]

Rick, Have to agree with jwd's sentiments.
However in your case you might be able to save/cure your problem with a bit of 'Kiwi DIY'.
Grab a magnifying glass about x10 power and have a close look at the 'dogs' on 3rd gear, work out which is the coast & drive sides of those dogs ( the drive side will have the most wear/coast less ), now you will probably find that the 'pointed' end of each dog is 'rounded' off, thats not really a major issue unless its really badly beaten up, but those drive/coast flanks can be, they should have a very slight taper lengthwise so that the dog gets narrower toward the gear, this helps hold the slider in gear. Those dogs can be dressed back to this condition with a small flat disc in a die grinder like dremel.
In order for this to work out new sliders may be reqd & I would strongly recommend the torq lock sliders mentioned by jwd as well, along with checking for mainshaft twist/bend etc. Also check for end float in the mainshaft bearing/midplate as this will interfere with slider engagement in the condition you have described. Out here in NZ we have a saloon series that used the super T10/Richmond etc box's with a straight cut/dog engagement gearset and a lot of weakness's showed up, especially around the mid/plate tail housing interface where we had to make up sleeve dowels etc to limit movement, even then there was evidence that the housing was twisting under load, Tex Racing have probably taken the T10 to its ultimate form with the T101A, but thats an out n out race box, even that may have been superceded by now, I have not kept up with nascar etc in recent times.