Since the tranny has to come out to the rear main seal, that's why it's lumped into that repair. You'll be paying for the labor to remove the tranny twice.
If you mean an axle seal from the side of the tranny, then that's different and much easier to replace, but I hope you have oil left in your tranny at this point. Although you say third gear is starting to slip, so I would assume it's automatic and you would know the color of the oil it's leaking. And since I don't have much experience with Honda automatics (yet), I'm not going to give any advice on flushing the fluid.
Why do you think the head gasket is going? Because of the oil leaking around it? It could be, although there are other spots for the oil to leak from and look like the head gasket area. How much oil are we talking about here? A half quart needed every 500 miles?

You CAN retap the threads on the pan (if needed) or try a new plug. I've seen some funky cures for fixing leaky plugs that I assumed the previous owner had stripped from a oil change place.

I would say to get a definite list of where the oil is coming from, then work out a plan of attack. Anyway you cut it, you'll have to do the tierods, alignment and tires.