Car won't start when I turn the key

so, i gave the starter a few taps, but it didn't want to work. i did a little more research and everything points to the battery being weak, which is odd since the car didn't have any problem starting hours before the problem started, and the engine cranked fast when it did start with the key.
Also, the space (or lack of) you have to work with is horrible, i might just take it to my mechanic and save me the hassle

If your battery is only 3 months old, it shouldn't be worn out, but it might be discharged.

My other car [non-Jimny] did have a faulty alternator on it, probably for years before the ignition light started flashing and wasn't charging the battery correctly, which caused the battery to wear out in about 18 months.

Your local garage should have the gear to test the battery and alternator (my little test meter does both) and also may be they can tell if the current drawn by the starter motor is normal or not.

Generally speaking, a weak or flat battery will not behave as described in your first post.

If the battery cannot deliver the cranking amps required by the starter one of three things will happen, the starter solenoid will engage and the engine will crank slowly, the starter solenoid will engage and the battery voltage will "collapse" with all the lights going out (not just dimming), this is usually accompanied by an audible "thunk" of the solenoid engaging, or the starter solenoid will engage, the battery voltage will collapse, the starter solenoid then drops out because of the voltage drop, the battery voltage recovers because the load of the starter has been removed, the solenoid then re-engages and this cycle repeats - thunk, thunk, thunk with almost machine gun rapidity.

I'll give you a 90% probability that your problem is the starter or the starter solenoid.

i'll get the car to my mechanic and have them check it, i'm sure it's the starter at this point though. thanks for the help guys