I live in Long Beach CA and have 3 established plumeria plants that need to go into the ground. I can't get pots that big!!

Since I have 3 (2 white and 1 red), how far apart should they be?

I was planning on aiming for March but reading through the other posts now I'm thinking maybe I should wait until May?

And what can anybody suggest in regards to soil preparation? Our yard is pretty neglected so I will be clearing out a rather large cactus-looking thing as well as a lot of over-grown grass and weeds (we moved into my grandmother's house). The basics, if possible? I'm not a big green thumb and admittedly don't really know what I'm doing. (It's just that my plants are beautiful and I love them and would like to keep them alive.)

Any assistance would be greatly appreciated! :)