If this is a serial-number activated LR just install on the new one and uninstall on the old one. There isn’t a formal activation count, or at least hasn’t been with the serial-number LR, so that is all you have to do.
If it is a CC-licensed LR+PS bundle, then when you install on the third computer, it’ll ask you to activate the other two, which you can do, then only ever log into one of the old ones, again. CC does keep track but makes it easy to transition to a new set of two computers without having to call Adobe.