I have got a desktop Mac and two Windows laptop PCs. If I subscribe to Lightroom CC will I be able to open it on all three devices?
the only limitation is that you must be signed-out from one of the three at all times. that's really no significant limitation for a single user.
You can install it on all 3 computers, can be signed in on two at a time, and can actually use it on one at a time.
If you want to use it on the third one, just sign out of one of the others and sign in on the third one.