That changes the situation. It is not generally considered good to combine the AGM and the FLA. You could move the AGM to the starter position and combine the other two batteries into a single
Otherwise, a separate charge controller is the correct answer. This does have an added benefit. The ideal setup for solar panels is a separate charge controller for each panel, allowing the system to adapt to uneven shadowing. If you set it up as two separate systems, and then in the future combine the batteries, you will already be positioned with two charge controllers, the ideal solution.
300Ah total isn't a lot of power
. What is your boat?