1.5ohms between 17 to 25watts, for that range I'd come down to about 1 to 1.2ohms
Example 1.26ohms on a single battery Evic VTC Mini, at 17.5watts is 4.7v needed, 25watts you are needing 5.61v or abouts, but in a dual battery mod like that the mod is having to buck, or shunt down the power being supplied by the batteries, 6.4v lowest and 8.4v highest fresh charge, triple battery you are looking at 9.6v and 12.6v needing to be bucked down to what is required. May not be the batteries getting warm but the control board and mod needing to buck down that much power, I can see the batteries getting warm if you are using the boosting circuit which takes extra amps from the battery and converts to extra volts when voltage being asked is higher than what the batteries have to supply, pull more amps batteries get warm. So maybe looking into a single battery mod would suit you better in the long run, as stated I use several single battery mods for my low wattage applications, that includes my Evic VTC Mini (max 75watts but run between 8 to 35watts most times), Kbox Mini (top tank kit, also a 70ish watt mod), Kbox 70watt, and an older Sigelei Mini 30watt.
Single battery safety under boosting to your wattage ranges on the LG HG2 which factory spec is 20amps CDR, independent tested by Mooch they are 18amp CDR
17.5watts/3.2v lowest charge=5.46875/90% efficiency=6.0764 actual amps needed
25watts/3.2v=7.8125/90%=8.6805 actual amps needed
both applications are well below the 50% CDR available
The only benefit I see running dual/triple battery mods at such low wattages is the extra watt hours, though higher CDR batteries you'd still see a heat up, heat conducts just like voltage, control board gets warm, mod and batteries will get warm, atomizer/tank gets hot, the heat conducts down into the mod, into the control board and batteries, just the constant current if you chain vape causes heat in the control board and mod.
Mooch a lot of times in interviews says internal temperature of the batteries gets damage at the 80 Centigrade/Celcius (176F) which I doubt you are even getting close to, thermal runaway temperature is in the 120C to 160C range where it can not be contained (248F to 320F), in either case if the batteries were getting hot enough to worry about you wouldn't be able to touch them without blistering your fingers.
Your style of vaping you need Mah more than you need CDR, the HG2, 30Q, and VTC6 all fill that role perfectly as well as gives you the needed CDR (18 to 20amps).
Guessing here, thinking about how many charge cycles you have actually done in the past 6 months on those HG2, guessing averaging maybe 3 to 4 charges a week if that, 6months X 4 weeks = 24, 3 charges per week X 24 weeks = 72 charges max on at least 2 of those batteries, to 96 charges at least on 2 of them, so only 1/3 their half charge cycles burnt down, full charge cycles of about 250 to 300, half charge cycles which most regulated mods do, that is 500 to 600 1/2 cycles, still plenty of life in those batteries.
Only safety concern I see, is the fact you state you have bounced between dual and triple battery modes, with more use on at least 2 of those batteries, the set is not balanced, no longer married, only 2 can safely be said to be balanced, the 2 used the most, that lonely 3rd is stronger than the other 2 being more fresh and less wear on it, there in a battery log book would tell and help in the rotation to get the batteries back into balance as a 3pc set. Marrying batteries is just like IRL between people, staying together until death do they part. Married batteries, bought together, charged together, used together until death do they part, moment they are not all used/charged together the set has no longer been monogomous to their mates.
Suggested batteries if you are looking at a new set, Samsung 30Q, Sony VTC6, LG HG2 in that order as best to lowest CDR, less mah but better overall all rounders Sony VTC5A (2500mah 25amp CDR), Samsung 25R, LG HE4, Sony VTC4, Sony VTC5 (2600mah 20amp CDR) (be careful when looking at the VTC5 and VTC5A, they are completely different, the VTC5 is the older model with 100 more mah and 5CDR less, the VTC5A is newer with 100mah less and 5CDR more, there mah and CDR specs are in parenthesis).
