How to charge 3 12V batteries with 2 24V solar panels

but might 36v from a pair of panels damage the actuator circuitry?

So here's the deal. Lead-acid batteries look electrically like a voltage source/sink with a small series resistance, with the voltage level a function of state of charge. 2V/cell (there are 6 cells in series in a 12V battery) is nominal, and if I remember right, their open circuit voltage is something like 1.9V empty, 2.1V full. That covers 90% of their behavior.

Considering that, the "1W@18V" spec of the solar panel isn't going to be able to "win" against the battery, and the solar panel's voltage will be pulled down to battery voltage, delivering probably 0.055A (=1W/18V) at whatever the battery voltage is.

When a battery gets completely full, however, its series resistance goes up dramatically, and the voltage goes up, until there's enough voltage to start electrolysis of the fluid and you get H2 and O2 generation at the terminals and loss of the electrolyte. A lead-acid battery, depending on the type + manufacturer, has a certain recombination rate of H2 + O2 => electrolyte that it can handle; if you electrolyze at a higher current than that, it leads to permanent electrolyte loss (+hence capacity loss)

So there is a safe current that can be delivered to a lead-acid battery continuously, where its own self discharge due to electrolysis balances the charging current. It depends on the manufacture + construction. I wouldn't feel worried about a C/10 or C/20 rate of charge (where C = the current needed to discharge a battery in 1 hour). Garage door batteries are probably > 1Ah capacity so you should be safe with 55mA charging current.

HOWEVER -- I would probably put a (zener diode and resistor in series) in parallel with each battery, the zener diode being about 14V and resistor being maybe 10 ohms or so, so that it keeps the battery terminals from getting charged too far.

Also: if you can, wire each solar panel to each battery (and keep the diodes), rather than the pair of panels in series wired to the batteries in series -- i.e. try to connect the center taps. By doing so, you'll charge each battery independently. Otherwise, what can ruin battery life is if the battery voltages diverge -- the one with the higher voltage will tend to get overcharged, while the other one will tend to get overdischarged and not completely charged.

Yes, it's okay, on condition that you never let the battery voltage exceed a maximum.

The maximum is typically around 14.4V to 14.8V at 21C temperature. If the maximum is exceeded for minutes to hours, the battery will be permanently damaged. The damage occurs around the same time that flammable gas is generated and vented, so there is a risk of gas explosion near the battery.

Your setup would require constant monitoring, especially if the load is disconnected or turned off. Check the voltage every hour, and disconnect the solar panel as the voltage approaches the maximum.

To find the maximum voltage, check the manufacturer's label or datasheet for the battery.

You can make your setup safer by adding a charge controller to protect the battery (so you don't have to monitor), by ensuring there is air flow, by removing any sources of ignition for the gas, and by keeping the battery away from things it might damage as a result of an explosion.