Yes. You should ensure that your array cable can carry the short circuit current of all 3 strings continuously. You should also ensure that your array cable does not drop more than 3% of the maximimum-power voltage, at the maximum-power current.
So 4 mm² will be OK if your most distant panel is no more than 8 metres by cable length, from your inverter. 6 mm² will be good for 12 metres. If it's further than that, you could run each string to the inverter in a separate 4 mm² pair. That would be good for 25 metres.
You never need string fuses when paralleling two strings, but when paralleling 3 strings you need string fuses if the short circuit current from two strings in parallel is greater than the maximum fuse rating given in the panel datasheet.
Probably not, as it should not be carrying any current at the times when it opens or closes.Is the constant relay clicking having any significant impact on lifespan of relay?
No. There will be no problem in winter with strings of 2 x 72-cell panels, no matter how cold it gets. But if you leave it with strings of 3, the problem will only get worse as the weather gets colder.Its Summer now, will changing string length affect winter situation?
Thanks again for you response
The consideration with strings of 2 x 72-cells is whether, in the middle of the hottest summer day, their maximum power point voltage will fall below the absorb voltage setting  plus 1.5 volts. This is very unlikely. And even if it does occur, it only means that you will not be getting the maximum power from your panels at that time. These are times when excessive voltage drop in the array cable can have a significant impact.