All you need is a good spreadsheet. The focus is the €42,560 and how much extra interest you'll pay by keeping it variable.
First, calculate your minimum monthly repayment (this is made up of interest and capital) - the PMT formula will do this - this is effectively what you're direct debit would be. In your case your monthly repayment @2.95% should be €182. Remember only focusing on the variable rate element.
Second, what will the outstanding mortgage be in 3 years time (assuming the alternative option is to fix for 3 years). The FV (future value) function will do this. Again using your numbers your €42,560 can expect to decline to €39,647
How much interest will you pay? You'll pay a total of 36*PMT and your mortgage will reduce by €42,560-FV. the difference between these two numbers is your interest expense. For you €3,643 (A)
Redo the above using the fixed rate interest rate and you'll get a different, PMT, FV. From them calculate your alternative interest expense = €3,017 (B). In other words you'll pay an extra €626 (A-B) for the pleasure of staying variable.
The above assumes you do nothing other than pay the minimum. However, you can tweak your monthly repayment (PMT) that you used in your original FV formula. By doing this you will pay more and in turn accrue less interest. Trial and error is the best bet here. You'll eventually land at a number €575. That number produces an interest expense, over 3 years, equal to €3,017 i.e., this is the amount you'd have to commit to pay each month to leave you no worse off than if you'd fixed.
Looking at the big picture, fix it all and you'll be paying €855. Leave 20% variable and you'll want to be paying more than €1,259 a month towards your mortgage to make the variable worthwhile.