After a recent secondhand boat purchase I sought to upgrade the older solar install. To this end I reached out to four different vendors, not initially intending to reach out to so many but after the first vendor obviously had no idea about my wishes I moved on to the second, who appeared to understand just what I was wanting to achieve and offered to email me prices and availability etc, crickets, no response over a week later, so onward ever onward I tried retailer number three, same parameters but I sensed that the woman that answered the phone either failed to understand or perhaps just wasn't interested. Once again no response after waiting so I tried to contact victron direct to establish if what I am wanting to do is reasonably viable, no phone contact available so sent an email to their published address, no response. The email didn't bounce back, I wasn't wishing to purchase through them, just needed product guidance.
On to vendor number four, one that I had previously bought from and after explaining my wishes they offered guidance and promised a detailed quote, which arrived in my inbox within the hour complete with options and prices. Thanks Preston offgrid, a business that I have to recommend and would happily use again.
Oh, the task? how to connect the start battery from each motor and the house bank to one solar controller in such a way that allows the solar to charge all three batteries as needed while allowing the engines to charge the house while they are running without interconnecting all three batteries permanently.

Good to hear POG came through. I have found them helpful too.

Good on Preston Offgrid!
Sadly, it's not just retail you see this. I'm working on a project where I put together a request for quote for about $300K of kit and sent it off to three big end of town Oz suppliers of said kit. Two weeks of nothing.
Sent it off to some smaller mobs, all responded.
When word leaked out the total hardware package is about $3 million, the same big end of town mobs are now beating down the door wanting to be besties. I politely told them they're out of the running. The littler guys are doing great, and for me, pre and post sales service is more important than the name on the shirt.

RE Oh, the task? how to connect the start battery from each motor and the house bank to one solar controller in such a way that allows the solar to charge all three batteries as needed while allowing the engines to charge the house while they are running without interconnecting all three batteries permanently.

DON'T EVEN TRY.
This can be done quite simply.
By "each motor" I take that you are on a cat.
Engine 1 = Charge start battery 1 and house battery through an autimatic charge selector
Mine looks like this.

Engine 2 = Charge start battery 2.

Solar panel has alligator clips so it can be connected to start battery 1, start battery 2 or house.

Then have a jumperlead that is long enough to connect between
: start battery 1 and house.
: start battery 2 and house
: start battery 1 and start battery 2.

Also this is the type of voltage indicator to go for.

Gary

Corrected wiring diagram.
Can you spot the difference?