Okay, so each camera draws 2 Amps. I have a feeling that is MUCH higher than actual but that's okay. I'll come back to this.

So each camera uses 567 Watt Hours per day. Because:

2 Amps x 12 Volts = 24 Watts, and 24 Watts x 24 Hours = 567 Watt Hours.

You have 3 cameras, so you use a total of 1,728 Watt Hours per day.

Normally you DON'T want to drain your battery bank more than 50% in terms of Amp Hour capacity. There are different ways to spec the AH capacity for batteries, but for now we'll assume the 20 hour rate. Also, many people don't want to discharge more than 25% but for now we'll use 50%.

We'll assume (for now) that you want the batteries to last one full day without being discharged more than 50%.

So your battery bank must have a capacity of 3,546 Watt Hours, because

1,728 / 50% = 3,546

Assuming a 12V battery system, 3,546 Watt Hours = 288 Amp Hours (AH). Which, I'll be honest, is a lot. It will take more than a couple deep cycle batteries to get that much power.

But, if you have (2) 12V batteries wired in parallel, and each battery was 150 AH, you'd have a 300AH battery bank and it would do the job.

But,

I DON'T think your cameras will actually draw as much as 2 Amps continuously. That is going to be the max amount of amps they draw, and they might never actually draw that much power. Which means you will actually have extra power stored if you use the numbers I gave you above.

You can't use a Killawatt Meter to find the actual power draw because they only work on 120VAC. But you could get something like this:

https://www.amazon.com/dp/B01LVTST80
to find the actual power draw and get a more accurate number to calculate with. I've used one similar before. You just wire them in series with the device and they'll give you a number. Let it run for a full day and get the Watt Hours used.

Then again, maybe I'm wrong and they actually do use that much power. If they're constantly uploading pictures, maybe so.

.