i have the 3pm fm tx. i have an hpi r40 (for racing) and a savage. is it a good idea to upgrade to a 3 pk in my situation?

Depends, are you heavy into racing with the R40? Will you race more cars? Do you need more setup choices than you have now with the 3PM? If number 1 is a yes, then buy it. If number 2 & 3 are yes then buy it. Basically if, 2 of those are a yes, then get one.

will the 3pk work with an FM HRS reciever? im asking because i have one in one of my other vehicles.

yes

and is the reciever that the 3pks comes with better than the regular HRS?

not sure what the new 3pks come with, I bought the older one used from someone
I use the synthesized module with novak synthesized receivers and its fast enough

is 2.4 ghz better than HRS? is it faster? is the range farther? is there a wire antenna on the reciever? can i use the same tx for 1 model that has HRS and another model that has 2.4ghz? is it good on cars and trucks? (sorry for all of the questions)