Welcome to the group pnut and hope you like the V3!
You can use either button top or flat top batteries in the V3 and V5. I prefer flat top IMR batteries and use mostly the Efest V1 IMR 18500. I also use 18650 a lot at home. Once in a while I use 18350 if I want the Sigelei V3 at it's smallest length.
For Efest IMR the V1 is the flat top and V2 is button top. The only difference between the two is that the positive terminal connection is about 1 mm taller on the button tops.
I usually buy Efest IMR at Crystal Clear Vaping. The flat top V1 cost less.
Efest Batteries
AW is another good IMR brand. These will perform better than the Efests at the upper wattage range of the Sigelei. I vape around 7.0 watts and find the Efest IMRs to be my best buy.
Genuine AW Batteries at RTD Vapor
I hear that some mods can not use flat top batteries. The Provari is the only one I know of. Maybe the Provari uses a mechanical fitting for button tops to achieve reverse battery protection. Sigelei doesn't need to as it uses electronics for reverse battery protection. I have no plans on buying a Provari in the future so I take advantage of flat top batts.