Yes, they are, for the most part, completely compatible. Parameters that only exist on the FX3 will be ignored and (silently) hidden.
There's an option in Edit to add the blocks in a preset to the Block Library: Preset > Save All Blocks to Library. That'll cause a sub-folder of the Blocks Library folder to be created which contains the blocks for that preset, making it easy to move or delete them en masse if you want later. You'll need Axe-Edit to break the preset down to blocks though, which also means you'll need the FX3.
I generally don't dump all the blocks like that though, I'll selectively pick a particular block I want to transfer, then save that to the library and give it a name like "FX3 blah blah" so its easy to find.
And, just so you know, when I move presets from my FX3 to the FM* units, I typically start on the FX3, reducing the preset until the CPU % is at most 50%, and the blocks themselves exist on my target machine, then export the preset, import it on either the FM9 or the FM3, and adjust further. After doing that on several different presets we get so we can do it pretty easily. That's basically the general path we follow when making/adjusting the factory presets; They start on the FX3, then get reduced to fit into the FM9 or FX3.
