Best way to cross-platform developer and build (for a solo indie)

Although I've released a few cross-platform apps, I have dealt with them simply on the whole, creating on one platform, copying code across, making changes if necessary, copying back and forth, because the apps were very simple. I have one collaboration where we used SourceTree but that screwed up on my PC and did more harm than good.

My next project is going to be cross-platform multiplayer developed by me in tandem on all platforms, with each step being all-platforms with networking tested cross-platform rather than progressing one platform and then porting the changes. Ideally I'll want some cloud-sync files. Development is on the Windows 10 workstation which can swap between Android and PC, with a port to a Mac Mini for iOS builds.

What are other people doing in this regards? SourceTree is the obvious tool but as my experience was dire, I'm loathe to trust it.