In use insulin should NOT be kept in the fridge so three days away should not be any different to three days at home.
Using insulin straight out of the fridge can cause bubble and is more likely to sting when you inject.
As
@AndBreathe mentions, if you are going somewhere hot, Frio wallets are the most common solution to keep insulin cool. However unless you are heading to the Tropics to spend hours out doors, insulin is fine at "Room temperature". Last year, I went to Dubai where it was 39 degrees outside. At those temperatures, I spent most of my time in air conditioning without using a fridge and forgot my Frio but my insulin was fine.
So the solution I would suggest that is "not too expensive" is
nothing.
My only additional recommendation is take spare supplies with you - extra insulin but also extra needles and test strips. Unfortunately, things can fail and they never fail at the most convenient time so make sure you have spare pens, etc.
Insulin can be kept out of the fridge for up to a month. Therefore, I would take a note of the spare insulin you take with you when you return home. It is absolutely fine to put back in the fridge after 3 days out but it may only last 25 days out of the fridge when you next use it.
But please, please, please stop keeping in use (opened) insulin in the fridge. It must be putting restrictions on your life having to remain close to a fridge all the time.