It’s important that I socialise in the evenings to network.
is it? these are already work colleagues....you already network with them. if drinking is the central activity, this truly is not a bona fide opportunity to network.
it's three days. you have the right to decline any invitation, you do not need to explain any decision. as Bimini said, a simple No Thank You is more than sufficient. walking away from anyone who wishes to press the issue is also an option. the only pressure here is really in your own head......