how do you go on to decide the filler? is there something that is considered the best 3 filler pokemon right now and are good against all common playstyles?
Filler pokemon should be able to check as many threats possible either with great type coverage, very powerful moves or some decent bulk to take hits. The best filler or "glue" pokemon combine all these three factors to cover as many threats as possible that your sweeper/team is struggling with.
Firstly, as BKC said Latias is probably the best filler mon out there since it can work in ANY playstyle and it can do a lot of damage with STAB LO Draco Meteor, with Surf and Psyshock rounding off the coverage. Plus, it contains Roost to take any necessary hits. It also counters pretty much the entirety of Sun and a whole bunch of Rain threats.
Secondly, Starmie is another great filler mon since it can not only spin to make your sweep much easier but with its great 2 or 3-move type coverage it can hit a very nice chunk of the metagame for super-effective damage, which is especially useful considering that Starmie outspeeds nearly every non scarfed or non boosted pokemon out there.
Finally, CB Scizor is probably is the most reliable filler mon out there since it can easily 2HKO any Pokemon with its Technitian boosted STAB Bullet Punch, regardless of its speed. It also has an equally powerful U-turn that can also scout for any switch-ins. Its typing finally gives it a whole load of resistances against many common pokemon and only one weakness.
Honorable mentions go to Latios, Terrakion, Garchomp, Rotom-W, Celebi and even Weavile :O