Not a pro, but have plumbed two houses. I'd do water heater first as it looks like it's a gas unit, and you may be limited by gas lines and exhaust location. Then I'd do manifold to make sure you have enough room for the runs. I built a manifold using copper exactly like
Just Needs Paint and found that I needed quite a bit of room for the pex bends. Finally, while placing these two I'd make sure that there is another room for the water softener, but that shouldn't be a problem. Keep in mind the flow of water, and try not to make things too complicated. For instance, try to make the water flow sequentially from one side of the room to the other and arrange the equipment in that order. Just makes a cleaner mechanical room.
Your hot water heater is vented to the outside, I assume, so your clearance needs are fairly minimal (I'd still try and get a 12-18 inches clearance all the way around):
Ernie Graves
I'm curious why you are using an expansion tank if your water heater is tankless, I don't think you need this?
2018 International Plumbing Code (IPC) section 608.3