Powering LED strips with batteries
That battery pack is advertised to supply 25800 mAh @ 5V.  Even though I am skeptical of those claims, let's use those numbers.

25800 mAh is 25.8 Ah.  That means it can supply an amp for 25.8 hours, or 25.8 amps for one hour (of course you can't really draw that much current).

A boost converter will step this up to 12 V, but consume current to do so.  let's use the ratio 12/5 to estimate our current needs.

$$3\ A * (12\div5) = 7.2\ A$$

What this means is that to draw 3 A @ 12 V, the 5 V side of the converter needs to draw at least 7.2 A (it will be more due to inefficiencies).

This is more current than a pack like that can supply.

If it could it would run for:
$$ 25.8\ Ah\div7.2\ A=3.58\ h$$

Lead-acid batteries are more suited to these levels.  Unfortunately they are often large and heavy, but they do make smaller ones.  Look into UPS batteries and evaluate their capacity the way I have shown here.

Example UPS battery

They do make rechargeable lithium packs in 12 V that might work for up to an hour.  Search for "12V rechargeable lithium".

Example lithium pack