The age old question of “how long will my battery last” is nearly impossible to answer definitively. There are just too many battery variations and environmental variables which all play a part in how long the battery lasts.
So, how long the battery will run some 12V DC powered device is more of a question for the manufacturer of the battery. There are so many thousands of batteries and brands that we just can’t keep up. If you have a 100 amp hour battery in theory you could run a 3 amp TV for 33.3 hours. But in reality drawing different amounts of power from any battery changes the actual time that it can perform.
There should be some kind of explanatory graph or chart that came with your battery/batteries that would help you understand the battery life with a 3amp draw.
You may also find that depletion rates change with various levels of charge.
In general, if they’re deep cycle batteries, if they’re good batteries and they’re big batteries, then you should be good for at least a weekend. This is no guarantee and you should refer to your battery manual.