I was planning to buy batteries for a RC Airplane which requires 22.2V of LiPo battery. There was the option to buy a single 6000mAh 22.2V LiPo battery but I figured that I could also use 3 7.4V 5000mAh batteries to receive the overall mAh to be 15000. Is this logic correct?
If yes, then how can the overall weight of 3 such batteries be less than that of a single 6000mAh battery? This is the 7.4V battery and this is the 22.2V battery.
3 7.4V batteries will need to be placed into series to reach 22.2V. When batteries are placed into series, their capacities don't add and you're left with basically a 22.2V 5000 mAh battery.
You would have to buy 9 batteries and put 3 of them in series and then put all 3 sets of those in parallel to get to 22.2V and 15000 mAh. You shouldn’t do this, though. To properly use multiple lithium batteries at once there should be some circuitry to balance the loads on the batteries and protect them. If the parallel cells aren't exactly the same voltage they will drain each other. Best case scenario this dramatically decreases the effective capacity and lifetime. Worst case scenario it starts a fire.
LiPo batteries are a pain to work with, you should NOT attempt any modification or grouping of cells, given your current electrical knowledge.
Those batteries can throw a MASSIVE hissyfit ending in a very real BOOM that could kill people nearby, in just a few seconds AFTER the shortest amount of time in a shorted circuit/bad contact.
Professionals are making those battery packs and include a lot of securities to prevent such events between cells of the same pack, you should NOT take it lightly. The energy density of those is nowhere near the old NiCd batteries (think at least 3 times higher).
Never short the pack itself, never try to make your own "pack" until you have sufficient knowledge.
Just to emphasize, we're talking CRATER IN THE GARDEN type of RUD (Rapid Unscheduled Disassembly) It's because of the way those battery terminals work, and even an "instant" shorting can start a chain reaction (look up for Thermal Runaway) that can propagate VERY quickly and in a matter of SECONDS make the whole battery go VERY bad.
Ask yourself if you prefer to have multiple packs and change them regularly, or if you prefer a single heavier pack you won't have to change.
if a small one goes bad, you might discard it easily and reduce/avoid damage to you, your car, or RC model (Think "live grenade", but crossed with a miniature cat hissing at you. GET those emergency release handles).
if a big one goes bad, exit the vehicle/area and run away screaming for cover.
Having a battery case seems like a very good idea with LiPo.
Just to reiterate over the actual danger, and give you some perspective : 1kg of LiPo battery (on full charge) is having the equivalent energy to about 230g of TNT.
The Mark-II "Pineapple" grenade, that you can see in every war movie, contains 52g of TNT. So your kilo of LiPo battery is about 4 and a half grenades... probably less confined than the cast iron body of a grenade, but those grenades weren't peppering you with lithium...
Your small battery is about 286g, including the wiring and plastics. I think it's about safe to assume 250g of it is the LiPo part.
Still more energy than a grenade.
Your big battery is 910g, still about 4 grenades...
Now you know, be careful with those, Never hesitate to ask a professional about it, There is no shame in being careful for yourself and others.
the battery capacity in parallel increases as a factor of the batteries mAh rating so 3 x 5000 (should be the same ratings) = 15,000 at 7.4volts (111 watt-hours). However, your question seems to be more series oriented - so 7.4v cells x 3 gives you 22.2volts at 5000mAh (still 111 watt-hours). It is important in a series combination that you should have battery management for balancing otherwise a weak cell will impact the whole group.
