Don't just buy 3. Buy 6, that way you can continue vaping your RX while the other set is on the charger.
Other good choices are the Samsung 25R, the Sony VTC4 (25 to 30 amp capacity), or the LG HB6 (30 amp).If you are going to push the watts limit of the RX you should go for the higher current batteries.
The mAH is how long the battery will last. The higher the longer it will last. Higher current batteries will not have the higher mAH due to the chemistry of the battery to produce higher current, but you can use more power.
Order your batteries at a reliable vendor such as Lion Wholesale. They only sell genuine batteries and test batches before selling them.
If you have the bucks invest in a standalone charger for your batteries. You can charge in the RX, but I trust an external charger more. Any that Lion Wholesale has are quality. $25 for an Efest LUC4 is reasonably price and a good charger.