Welcome to the forum.
I have moved your thread to Marine.
Charging 3 12V separately while in series is only common in the marine industry. You would likely need to look for a marine-specific solution.
Standard Solar PV and storage systems only charge the total voltage. Charging individual series components is rare enough that I've never heard of anyone doing it.
Many SCC can do 36V easily. IMHO, the solution *I* would come up with is a simple solar charge controller charging the 36V bank as a whole with 12V balancers working to keep them at the same voltage.
Will recommends several chargers here:
Building a vehicle mounted solar power system? Let me help.
www.mobile-solarpower.com
You'd need to confirm that they support 36V
I recommend and use the following balancers for my 48V bank:
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An important consideration is the amount of current the panel can provide.
Let's say your battery is a 50Ah battery. You would want to charge it with 10-15% or 5-7.5A. Since solar only products peak output at high noon on a perfect day, it's good to shoot for the high end. 7.5A * 14V = 105W, so a 100W panel should get you there. Note that when you put batteries in series, the Ah rating doesn't change, so 3X 50Ah batteries in series to make 36V still only has 50Ah.