In term of pvp I take it? Quoting myself in another thread:
The strength of the lancer path in the arena lies on his 3-tiles stab attack, especially in the current meta with tile control strat heavily abused. When using gladiator you can normally wait and hit the front enemy unit together with the range at the back. Gladiator is the most tanky out of all champion class and in fact is the second highest ATK stats champion if you consider his aura (losing only to berserker after aura kicks in). His stun, just like other cc, can be used to save his ally and delay enemy from casting skill. The addition barrier effect is quite helpful since he is always in front taking damage.
His downside is that his skill cant be used to OHKO the enemy unlike other 5* slayer path, which can be very crucial in the arena.
The other reason is that I dont have any other form of cc in my roster, so I build the Gladiator to pair up with my Dragoon for cc. One cc may not be enough for the harder contents. Everything else depends on your team composition.
Regarding Warlord and Berserker for pvp, I intend to make a Berserker soon too. Whats not to like about the best ATK buff and strongest active skill in the game?
As for Warlord, if you dont use other attack-trigger class like Grandranger, Gunslinger, Samurai, etc, I wont bother using him in arena.
Not sure about 3 champions formation myself, especially with the current meta focusing on tile control, the champions usually dont last long enough when placed in the middle tile. Alternatively, you can try focusing on all dps team with 3 champions + 2 rangers to see if that works.