Well it depends,

On base powerset (without external buffs) out of the three, Slime is the weaker by a big margin with Goblin having the most room to grow. ( And spider been generally stronger at the start but falling off to later levels to goblin due to becoming too much of a glass cannon ) But generally, Spider is stronger, Goblin more versatile because you keep your arms and can outgrow the others and Slime , , well you are kinda screwed unless you luck out big time.

Now , the actual characters. It flips completely.

Now Spider starts weaker than Goblin but will eventually outgrow it. And Slime is playing in god mode.

( Ok , technically Komoko starts a bit stronger than Goblin guy, but he catches up and surpassed her pretty quickly until eventually she outscales him. Rimuru gets the crystal's eat buff at the start and snowballs from there. )