If you were leaving the vehicle standing for a whole year, this would be a bigger problem, but I don’t think three months of inactivity would be too harmful to the car. Plenty of people (particularly Victorians) were forced to lay up their work cars during Covid restrictions, and I haven’t heard of mass problems as a result. You’re already ahead of some of those folks by having a garage in which to store the car, but there are also a few things you can do to reduce the impact of not moving for three months. Make sure the tyres are inflated to the correct pressure (to avoid flat posts when you return) and fill the fuel tank to reduce the chance of moisture forming in the fuel system. Wind the windows up but leave the bonnet catch undone so you can access the battery when you return. You can also disconnect the battery but if you do, you’ll have to reprogram or re-code the radio when you return. A better way is to leave the car hooked up to a quality battery trickle-charger. Make sure you use one that is heat and fault-protected. An even better plan of attack is to organise a trusted friend take the time to drive the car for you every few weeks. Provided they get the car up to operating temperature each time, it should be ready for action when you return.