Is this interrailing route do-able in 3 weeks?

doable ofcourse if you just travel by train without sigtseeing you can manage
on day one Paris - Brussels  - Amsterdam - Berlin
day 2 Berlin - Vienna - Venice
day3 Venice - Milan - Grindelwald
day4 Grindelwald - Geneva - Lyon - Perpignan - Barcelona
:D
It all depends how long you will stay in each City :)
Berlin offers atleast 3days (+ a daytrip to Potsdam you are on 4)
Vienna same 3days (+ a daytrip to Bratislava  = 4)
The rail connection between Mardrid and Lisbon is currently very bad (several changes needed and each train needs a reservation on top of the Railpass )

You start from the UK? why not start with a Eurostar to Brussels then with Reservation free Intercity (Instead of the expensive Thalys) to Amsterdam ;)

and then return from Mardrid via Lyon & Paris back to the UK ;)

I notice you’re from NI, so it might be worth pointing out that you aren’t actually time-limited if you need to take two days to transit to/from the continent via GB, because it won’t be considered your ‘home country’ for the purpose of this exercise. This is because Interrail separate Britain and Ireland according to geographical rather than political boundaries. (Unless of course I am incorrect about this, as has been known...) So if you take a day ferry to Liverpool or Holyhead, for instance, and then an evening train to London (or somewhere else along the way), you’re still able to overnight there and take a morning Eurostar the next day if that helps.