We just started a campaign. I started out as a a ranger and ended up taking 3 levels in it taking the gloomstaker archetype. I was thinking about multi classing into cleric possibly Tempst, war or forge domain. This is a good or bad idea?  

What do you want to have this character able to do? Does multiclassing help further this or are you just trying to min/max?

I have played a ranger before and it got a little bit  old doing the same things and I was looking for a way to use magic more in my build but still keep my long range attacks with the bonus with ranger. I could see the tempest cleric because of the abilities to control storms and weather or a forge cleric for the making of weapons. 

I think that is a decent idea. Tempest synergizes with Lightning Arrow. Forge gives you access to heavy armor. And either fit the theme of ranger.

I'd do Twilight Domain Cleric, if your DM allows UA. This would let you have infinite darkvision, and also be invisible to creatures that only have darkvision and normal vision in darkness.

While I love me some cleric, and multiclassing, and especially multiclassing into cleric. On a ranger there are other options above it. Which really hurts to say, because you are considering tempest cleric, and I love tempest cleric. 

I suppose first off before anything when a ranger multiclasses, are you going ranged or melee ranger? Because for Ranged, I could only suggest War, maybe tempest. While all 3 are viable in melee.  All 3 give you heavy armor, but again that is only useful if you going to use heavy armor.  Extra spells are nice, but the channel divinity uses are very limited.  So since you are going to stay ranged, War would probably be the best.  Your CDiv with Sharpshooter could be be a (pardon the pun) God send in a pinch against the BBEG. Tempest with a ranged weapon could do well for itself at first, but one maxed out call lightning and you might as well put a giant bullseye on yourself, because they will come for you. Outside of the maximized damage lightning or thunder with your Cdiv, and divine strike, a lot of tempest is more for being in the middle of the action, and getting hit.   

One I might suggest, for just it's level 1 ability alone for a ranged fighter.  Light domain.  Pop warding flare when targeted by anything within 30 feet to imposed disadvantage.  Not a bad little trick. Toss out faerie fire as your concentration spell to give you advantage on attacks.  Oh, and you get fireball. 

Quote from Houligan >>

I think that is a decent idea. Tempest synergizes with Lightning Arrow.

 Unfortunately, they need to go at least 9 levels ranger to get lightning arrow. An almost 50/50 split and only coming online late game.

I recommend going 5 levels ranger for extra attack and level 2 spells whatever else you do.