Seems to me a second hand bike costs about the same as a Triban 3 anyway.
that is true but decathlon have been very good with the warranty on our 2 triban 3's. I have had to have a number of parts replaced (primarily because I cycle all year round in all weather's and have worn out quite a few parts) and all have been replaced (and some upgraded) free of charge. Personally I would ring your closest store and talk to them. We have and have found them very helpful. The tyres are not that bad, not bad enough to worry about until you have worn them out. the pedals on the other hand are another matter.
We both have 2nd hand mtb's as well, and we have to pay for the parts replacement on them - luckily we have a reasonable idea on condition etc and were able to pick up 2 good mtb's 2nd hand.
Otherwise your only option would to be to rely on someone's honesty or a LBS who sells 2nd hand (again unless you know them you are relying on their honesty) or tell us where in the country you are to see if anyone here could visit a 2nd hand bike one day with you to assist with a judgement call on if it is worth the money etc.