Loosing 3 transmissions in a year is unusual. It's difficult to say why you are loosing them when I'm sitting on the other side of a computer screen.
Things like your driving habits, how the truck is used, quality of the rebuilds, what's been done to the truck, etc have a huge effect on the situ.
A stock 4L60e is a marginal transmission (at best) in a 5000-6000lb vehicle. Adding stress to it in the form of larger wheels and tires isn't going to help any.
Loosing 3rd gear repeatedly would indicate to me that you are fragging the 3rd clutches/bands.
There can be several reasons for this.
One is a crap rebuild. Not a lot you can do about that. If they don't check things like accumulator leakage and clutch plate warpage, you (unfortunately) won't know until it fails again. Rebuilds done at the local shop are usually done as cheaply as possible to increase their profit margin. There's actually nothing wrong with reusing certain parts, but some should be changed out even if they inspect serviceable (solenoids, accumulators, seals, etc). These are the parts that some times result in a "comeback" (for you, that means a failure).
Towing heavy and not shifting down from OD can frag the 3rd clutches. It's been a while, but I seem to remember the 3rd and 4th (IE: OD) clutches/bands work together in the 60e. Towing in OD frags 'em depending on the load or road.
Ripping on it hard in off road can frag the clutches.
Ripping on it hard on road can frag the clutches.
With a 4L60E (assuming it has no problems when it comes to me) I usually recommend they install an external cooler (if it doesn't already have one) and I up the line pressure a little bit in the calibration. This removes a lot of heat from the transmission and gives the clutches a fighting chance at survival.
I don't rebuild transmissions as a rule, but the 60E has lots of aftermarket "upgrade" options. They can be made to live behind decent HP and larger tires, but it takes a bit of money and parts to do it.
The other option is to swap in a 4L80E. I haven't done it, but it has been done before. Apparently, you can run the original PCM with some modifications. You'll have to google that info, I don't have it.
With clutch failure, heat is usually your enemy. Lockup behavior, line pressure and cooling are your friends in fighting heat.
Good luck.