Using an ISA to maximise savings on a 0% interest purchase. Am I being dumb?

I'm having to buy a new boiler for around £6000. This option with British Gas comes with a 36-month 0% interest purchase as an option.

I have the cash and could pay it out right, but would the smarter move be to continue paying that cash into an ISA savings account (or high-interest savings) over those 36 months?

Pay the first year out of cash via the monthly payments, but use the rest to save and gain interest, and as it matures, repeat the process again for the 3-year period?

This feels so like the right choice, but I always feel so dumb with these things and would rather have my savings working for me in a safe way rather than one and gone?