From the sound of the setup you will need at least 24v for the controller
You have stated that you have a bunch of 7.2v batteries
I assume by this voltage notation that you have Nimh batteries (1.2v cells in 6 cell packs)
This type of setup is not optimal but could be used
At full charge, NIMH cells read about 1.45v per cell which would give you 8.7v per pack
This means at full charge, 3 packs in series would give you 26.1v which would meet the requirements for a 24v setup
The next issue you have is capacity
3,000mAh is very low - Most people will run at least 10Ah or 10,000mAh - for your battery type this means you would need 3 sets of 3 batteries for 26.1v 9,000mAh or 3 sets of 4 batteries for 26.1v 12,000mAh
For testing purposes, the 9,000mAh will do
This means you take 3 batteries, connect them in series - 3 times
Leaving you with 3 different groups of 3 batteries at 26.1v (fully charged)
Then you connect these batteries in a parallel harness to combine the mAh to give you 9,000mAh
Then connect this to the controller
(depending on how it is setup, it may need to be programmed for a 24v setup)
-I too am from the RC world, I converted a 8ight-t nitro truggy to brushless electric, as well as many others
-If you are comfortable with RC setups, lipo is far more powerful
Let me know if I can help
Hope this was helpful
God Bless