Adding more solar panels

I would check if your existing panel is connected using MC4 connectors.   If it is and most are then you can get MC4 branch connectors you can do as you described basically plug and play.  Buy a MC4 connector tool a couple of £’s to make like easier.   I guess the only question is, with an mppt controller to you want to connect in series or parallel.  

Probably have to change regulator as won't be able to take extra power. Most fitted by manufacturers have a limit of about 150w. 

The regulator will have to be changed as I want to have a dual battery output plus a lithium setting, neither of which are on existing unit.

Yes no issue but a couple of points to note.

The new panels need a similar output voltage profile to the existing one and your feed cables may need to be upsized to accommodate extra current and minimize voltdrop, this should be no more than 3%. 

Assuming you have 300w generated at a constant 15v (possible scenario) then you would need 3.00mm2 cable between the juntion box of the panels and the controller and then you need to check volt drop.

If I wire in series, I'll be looking at close to 60V going to the controller.

I see, so strap 3 together and thus keep the current constant at higher voltage. You then step the 60v down to 12 in the controller. 60v is legally the highest yo can go but is allowed. 

I have no idea if you can get hardware to step from 60 to 12 but i see your logic. You are in spec for cable max voltage (just) for thin wall automotive cable.

Interesting project.

I think the positive of connecting in series is the reduced amps means the wiring diameter can be reduced and power loss is less. The potential negative (see what I did there) is that shading of parts of the panel has an increased deleterious effect on output.   There are a few videos showing this on YouTube. 

Have a look at the Victron Energy forum, there are many highly experienced solar engineers and Victron engineers to guide you.

3 in parallel will give you approximately 15 + amps current, you will be better of wiring in series that way you will have a maximum estimate voltage of 66 v and 5 amps current, it is the amp current which generates the heat.

I am running 2 80w watt solar panels in series (44v) to a Victron Smart MPPT 75/15 controller, you can set it to 24v or just leave it on 12 v setting.

For your 3 x 100w watt panels you will need 100/20 controller, have a look at there helpful forum, the can explain all your answer far better than us.