Just wanted to get some stuff ready and I know this has been done to death but what are some good enemies that a party of 3 can take with reasonable levels of difficulty. I know one set I want is to have a group of Kobold guards snoozing and the party gets the upper hand on. But I don't know what else to get from ancient prison/temple. Maybe a big Spider or single giant centipede or what?

What level is the party?

What classes?

Level 1 and they're showing up with the character sheets. Handbook Classes + Artificer only. I dunno what they'll bring

Use the encounter tool here to build challenges that match the power level of the group. It's intuitive, can be saved, and employes the encounter building rules from the books. That'll let you tinker and get a feel for what CR matches the difficulty you want.

It mostly depends on what you want in the ancient prison/temple. Are the Kobold guards actually guarding a lair of more Kobolds, or are they just a forward team that was also investigating the place and are out from or in the dungeon somewhere resting when the party gets the drop on them? If they are guarding a lair, more Kobolds and clever traps is likely the right answer. If they are exploring the dungeon as well, then you can slide in some evidence of their passage, maybe some looted areas, some dead monsters, or maybe a dead kobold or two in some of the traps. 

From there just decide what is already living in the dungeon. Some fun options include:

• {CR 1/8} Giant Rat, Gremishka, Manes, Stirge, Twig Blight
• {CR 1/4} Dretch, Geonid, Giant Wolf Spider, Needle Blight, Oblex Spawn, Skeleton, Violet Fungus, Wretched Sorrowsworn, Zombie
• {CR 1/2} Chitine, Cockatrice, Darkmantle, Gas Spore, Shadow, Skeletal Alchemist, Topi, Vine Blight

And if you need CR  or higher monsters to mark as a boss or something, just fine what you like and filter down through the lesser monsters from there. 

Stirges never disappoint. maybe some unfriendly humans for some roleplay

Quote from Metamongoose >>

It mostly depends on what you want in the ancient prison/temple. Are the Kobold guards actually guarding a lair of more Kobolds, or are they just a forward team that was also investigating the place and are out from or in the dungeon somewhere resting when the party gets the drop on them? If they are guarding a lair, more Kobolds and clever traps is likely the right answer. If they are exploring the dungeon as well, then you can slide in some evidence of their passage, maybe some looted areas, some dead monsters, or maybe a dead kobold or two in some of the traps. 

From there just decide what is already living in the dungeon. Some fun options include:

• {CR 1/8} Giant Rat, Gremishka, Manes, Stirge, Twig Blight
• {CR 1/4} Dretch, Geonid, Giant Wolf Spider, Needle Blight, Oblex Spawn, Skeleton, Violet Fungus, Wretched Sorrowsworn, Zombie
• {CR 1/2} Chitine, Cockatrice, Darkmantle, Gas Spore, Shadow, Skeletal Alchemist, Topi, Vine Blight

And if you need CR  or higher monsters to mark as a boss or something, just fine what you like and filter down through the lesser monsters from there. 

 In this case it's more like they work for the BBEGs. They work as guards part time for gold.

maybe the kobalds are worshipping a wrymling? their pretty low CR