Congrats, Liam........and welcome to the triban 3 owners club!
How well I remember the thrill of opening mi giant cardboard box in August. An did you get the free choccy bars and broken lights? And the cheesy little bell?
I think you'll love it.........and even if you don't-don't forget it only cost you 300 smackers and everything is upgradable and adjustable.
I changed tyres, stem and pedals fairly quickly. You will find that the B'Twin 'fast, reliable, comfortable' Tyres are actually none of these things at all....but after you discover they puncture if you sneeze on them it is a quick and worthwhile fix. For everything else...well-it's all pretty serviceable stuff and amazing for the money.
I'm 5'8'' and plumped for the 54cm size which fits me well. There is plenty of wriggle room to adjust things to taste between the sizes so don't fret too much. My new stem made a significant difference for me and only cost beans.
Have fun!
Best
w