Strange: I'm Dutch and the FC3 is a Dutch product. I have had many of these cartridges but I neither heard of "pro" or "plus" version. There were different version because they would display a date when looking at the info screen. And AFAIR I had at least five cartridges with a different date. I cannot check that anymore because I gave the older ones away at the club.
The FC3 won't work in C128 mode because, when inserted, the C128 will start up in C64 mode automatically.
Yes, you can put the ROM in the empty socket of a 17xx for the simple reason it will fit. But if you can read it or run it, I have no idea. But what about using U36 in your C128