We’ll as I see it and have my boat set-up, the biggest nicest one goes on the dash. I run a Lowerance 111 on the Dash and you can clearly see if from any point of the dash and back, works great for the guy fishing the on rear platform. I run my smaller unit up front (lowrance 337).
I find this works good for me for several reasons, first off most of the time I spend really looking and watch the graph, I’m going to be in the driver’s seat, watching depth and or for fish, I also like that I have a nice big display for my lakemap chips. It’s easier for me to read quickly because of it’s size, and that comes in handy when you are driving on plane, I don’t want to be looking down trying to read something, I want to glance down and get the info I need, and get my eyes back up where they should be.
Secondly I don’t find I really need to moniter the bow unit as close, mostly I’m watching depth, I’m usually in the spot I want to be at that point, depth is more of a judgement that I’m staying where I want or need to be, and I’m never moving more than a couple MPH at any given time up there so size doesn’t have much benefit. I always have plenty of time to look a bit closer if I need to.
I also find RAM mounts to be very helpful, it’ll let me adjust the unit better to the light conditions, you can move it enough to get it out of the sun a lot better, it also lifts up my units so they are close to me, so my 337 on the bow sits up a good 6 inches high than if it were flush mounted, again much easier to read. I also run the big ball RAM mount on both units, so if it ever was an issue, I always have the option of swapping a unit. Though I’ll agree it is a PITA.
Put a big one on the dash, and I don’t think you need three, wiring would be much easier, and less transducers (unless you ran a switch) but it’ll probably cost you the same if not more. Cost wise, it’s cheapist to move the units around. You’ll need a second set of wires and either another transducer or a switch.