So I bought a few letters light that works with 2X AAA batteries and i am trying to connect them together to one AC source instead of using batteries. I opened one of them and there is nothing but the lights and the power on/off switch. Can I connect all the positives together and all negatives together? if so, how much AC power do I need? is it 3V or the total of the used batteries? I have 14 letters that's mean I need 28 batteries.
Interesting question: Yes, you can connect all the letters in parallel. But be careful here. You're talking about both AC (mains powers) and DC (batteries). Measuring DC current is more difficult than AC. with AC you can just put a DM with an amp clamp around a wire and whamo samo, you have a current reading. Measuring DC current is less common, possible, but just less common requiring a specialized meter.
Also, your power supply must convert from mains power to 3 volt DC, preferably with some capacitors to smooth out the supply. your lights might not work right or flicker without a high end power supply.
you can connect the letters in parallel to a single 3V supply.
The current requirement is going to add up so I can't guess how many amps you will need. is there anything written on the lamps? (if they unscrew on the metal part)
You could use a multimeter to measure how much each letter needs (I expect that I uses less than W) and add those up.