at a later date, when i have 3 sites running, each with lots of
content on, can i combine them at that time?
Yes, you can. I’ve found the easiest way to do so is to use the Export/Import feature, but it really depends on the site, and sometimes you have to get creative. Assuming these will be easy, what you would do is setup a multisite install, create new sites within this for each of your 3 solo sites, export XML files from your solo sites, then import those XML files respectively into the new sites in your Multisite network.
Does that make sense?