I will second @pmags’s advice about considering alternatives (for what this will cost, you can get an iPad, a knobby/slidey MIDI controller, and many great, flexible FX apps) and @KleinKlang’s advice about going slowly.
But let us assume your heart is set on these three (understandable; I have no need for them, yet I have been following their development and watching the demos). They total 60hp, and you have 84hp to work with. That leaves 24hp for everything else. Since you have no other Eurorack, and no space for anything more, this is a precious resource and should be carefully thought out. You don’t have the luxury of trying something and then shunting it off to another case if it doesn’t quite work for this purpose, or living with something that is a little too large for what it delivers.
Modulation has been mentioned, though there is some internal modulation in each of the core modules. You probably need to work with the FX modules with just your fingers and what they provide, first, to get a sense of what you are lacking. Strategies for modulating FX vary. I hardly ever modulate reverbs, though I will adjust space/delay on the fly. Others really get into it, to the extent of making the reverb a sound source. I have the Quad LFO that @pmags mentioned, though mine is paired with Acid Rain Junction. But this combo won’t do synced LFOs. For that I have Pam’s and Neo Trinity, both 8hp. Pam’s is more set-and-forget, and NT more playable (though, with no screen, there are more button combos to remember). Then at 12hp I have the more sophisticated Clank Chaos, a Marbles clone, and the new O.R.N.8 variant of Ornament and Crime. Each of these offer a different set of features, but they may be overkill in this case. There are a couple of more recent modules I’m contemplating, but I have no experience with them yet, so won’t speculate.
The Audio I/O takes up 10hp. You can get one stereo channel of I/O in 2hp with the Pindsvik LIHO (I have one of these and will get another). It’s well-designed and well-built. Audio I/O has a bunch of features intended for use with Intellijel cases and 1U modules, which would be wasted on you. Unless you really need dual balanced channels, I think you could claw back some space here.
Your proposed setup has no in-rack mixing except for the start-of-chain and end-of-chain provided by Audio I/O. I think a small stereo mixer would allow for some parallel processing and more flexible configurations among the three core modules.
I agree with the comments above about the redundancy of MFX and the difficulty of using Quadrax (I own this), which is similar to my issues with Neo Trinity. You need to be using these regularly to keep on top of them, and even then, you are not going to commit the full range of Quadrax’s complex LFOs to memory. You’ll probably need to print a cheat sheet for handy reference.
Here is what I suggest. Start with Sealegs and LIHO or another 2-4hp I/O module of your choice. When you feel you have a good handle on using Sealegs, add Swells. After some experience with both, come back here and tell us what you’ve learned. You may already know what combo of modulation and mixing makes sense at that point, or we may be able to help with that. Then fill out the rest of the case.
[Edit: tagging @crevice because I didn’t reply properly.]