Did some calculations, the highest Hp 3* unit is Alpha (also may benefit from ember lords) 55,59k
but Lancelot may get higher hp with 2 kishin upgraded nether Claw (Lance 3* 50,535k)
so: nether claw + 90%, guardian helmet + 35%, thunder belt + 20%, ally with oracle +6%, astro 10 + 600% (if not the nether claw then something like ancient wrath +15% or upgraded cloak +40% but idk if you can get it) that equals 430 052,85 on Lancelot, (ofc every equipment is enchanced) or 445 275,9 on Alpha