1) No, el coste de maná convertido del Goblin Heelcutter es siempre 4.

2) Sí, el coste de maná de Mardu Shadowspear es siempre 1.

3) Sí, el coste de maná de Nighthowler es siempre 3.

Eidolon of the Great Revel
RR
Enchantment Creature -- Spirit
2/2
Whenever a player casts a spell with converted mana cost 3 or less, Eidolon of the Great Revel deals 2 damage to that player.

Goblin Heelcutter
3R
Creature -- Goblin Berserker
3/2
Whenever Goblin Heelcutter attacks, target creature can't block this turn.
Dash {2}{R} (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)

Mardu Shadowspear
B
Creature -- Human Warrior
1/1
Whenever Mardu Shadowspear attacks, each opponent loses 1 life.
Dash {1}{B} (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)

Nighthowler
1BB
Enchantment Creature -- Horror
0/0
Bestow {2}{B}{B} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Nighthowler and enchanted creature each get +X/+X, where X is the number of creature cards in all graveyards.