/bossbar add a "The Archer"
/bossbar set a visible true
/bossbar set a players @a
/execute store result bossbar a max run data get entity b c
/bossbar set a value 0
/bossbar set a max 100
/bossbar set a value 75

Look up the boss health bar title shows as The Archer and percentage shows up with zero percent.

/bossbar set a value 100

/bossbar add b "avoma"
/bossbar add c "Nuclear Creeper"
/bossbar set b value 1
/bossbar set b max 1
/bossbar set c value 1
/bossbar set c max 1
/bossbar set b style notched_20
/bossbar set c style notched_20
/bossbar set b visible true
/bossbar set c visible true
/bossbar set b players @a
/bossbar set c players @a

Look up the boss bar title shows as Nuclear Creeper with boss health bar with notched styles has been messed up. 2nd boss bar looks like the LED Dot.