If I purchase the 1 enterprise + 2 professional package, can 1 of the 3 computers be a Mac and the others be PCs? Essentially, I'm concerned about mixing a combination of Macs and PCs moving forward and as my firm grows (I hope!), and whether there will be any compatibility issues or other problems with the two types of systems interfacing with one another?

That is not a problem at all. You can save your project files on a Mc and then open them on a PC or vice verse without any issues.