After activating “Delta Attacker”, you have 3 different monsters with the same name, “Delta Attacker” still only applies to the 3 monsters of the same name you had when you activated “Delta Attacker”.
If 1 of the 3 monsters is removed from the field, “Delta Attacker”’s effect still applies to the other 2.
If “Non-Spellcasting Area” is activated after “Delta Attacker”, the effect of “Delta Attacker” is negated.