Personally, for short journeys, I find there isn’t much difference between 1, 2 or 3 AC. It’s the same quality of beds and air conditioning. Plus, if you’re on during the day everyone usually sits on the bottom seats anyway.
A 3 AC carriage means that there’s 3 tiered bed in each compartment, on each side!
If you’re travelling overnight I would recommend 1 AC, the quality of food and bedding is just better and you (fingers crossed) may have more chance of sleeping! You get nice curtains for privacy, a private fan in the compartment and charging point.
2 AC (or 2 tier) is also fine and has two beds either side of the compartment like bunk beds. Curtains are present here but no private fan. There is shared charging point.
3 AC has 3 tier beds meaning 3 beds either side of the compartment. Best to get a top bed if you want to sleep in the day as the middle and bottom bed won’t be available until everyone wants to go to sleep. No curtains are here so no privacy either. And more people means way more luggage everywhere.
My favourite is ‘side lower’ as there is just a single bunk on the top and below. So you can chill out behind the curtain and just look at the world outside!
The bedding you get is pretty tired and worn out and the best not to look at the colour of the pillow underneath the cover! You should get new ones but I’m not sure if they change them in between stops for new passengers. So bring your own travel pillow and a blanket I say.
The Shatabdi express trains are the best for day trips. They’re more expensive but You get AC, 2 meals of decent food and a comfy seat. Booking chair car or executive class, there’s little difference. So if you needed to save a few rupees just book chair car.