No you won't get fined or anything,if you've not been sick,then you've not made any claims, so everything will be fine.

The worst that could happen is,( but I doubt it, ), you'll be made to pay back three months of insurance, but as you've not signed up, who'll know other than the council,and as you've not signed up, there's no missing premium to have a court order at you for non payment of premium.
Court order(BETREIBUNG)

Anyway when you sign up just make the start date from that day, and don't say too much.

Regarding the premium if you are healthy,and never really go to the doc. you may be better taking a higher franchise as this reduces the premium quite a bit, and make sure when you take out the insurance you know exactly what your covered for.
Sometimes it can be very clear and other times not, eg. if your not carefull you can be covered twice, ie. unfall( ACCIDENT INS.).
your covered by your firm compulsory, then why bother paying extra in your krankenkasse, IN SOME CASES YOU MUST PAY AN EXTRA PREMIUM, OTHERS IT IS INCLUDED IN THE PREMIUM but these are things to discuss with your insurer, so it's better to ask plenty if your not sure ,however on the whole the krankenkassen insurers are pretty helpfull.