I have a laptop with 11.1v battery and 19v charger voltage, is it safe to use 3 sealed lead-acid 6v batteries in series? And if I will to use 6 batteries to achieve 18v, is it better to connect each 2 in parallel, then in series the 3 packs of 2? Or each 3 in series, then in parallel both packs of 3
Be careful with those experiments! A Lead-Acid battery of 6V has up to ~7V when fully charged.
If you want to connect batteries in series or in parallel, you need to have the same type for all of them. Otherwise, they won't discharge and charge the same way and can get damaged.