i use these apps like 99.9% of my screen usage: Whatsapp, Telegram, Youtube, Chrome, Vinyl music player, and Clash of Clans.First of all I want to know what kind of apps you are using. I can't believe how u are getting only 3 hours SOT.
If you want to switch to another kerner then you can just boot to recovery and flash this No. 912 OSS Battery variant kernel.
For backup you can go to recovery and take backup of boot or just dirty flash the rom zip and you will get the perf+ kernel back.
i aso have Onedrive and Google Photos and Drive sync running in the background...i used the same apps on A10 ROMs and battery was far better....clearly there's something wrong with this ROM, but you're saying battery is good in your case so i really don't know what's goin on but you should try and figure it out... i mean there must be something different in your device or something
Edit: btw 3 hours SOT is the maximum i'm getting cuz it's almost draining the whole battery on idle...it's truly terrible knowing how good the Mi A3 battery can be