I am hoping to keep this question quite focused so I will set the scene: 

* I am starting my first D&D 5e campaign
* We are starting at level 3
* There is no rogue in the party so I am looking to absorb that role (No 'tank' either but hopefully that isn't an issue)
* The others in the party are a Warlock of some kind, a Cleric of the Death Domain and a Duel-Wield Fighter
* I put equal focus on being good in combat (Controller) and out of combat (Face / Skillmonkey)
* I am set on being a Lore Bard, taking the Criminal background to get proficiency in Thieves Tools (Not married to the specific background however)

What I am looking to do with my build is multi-class (Single level dip) to try and add some extra effectiveness. 

Cleric was recommended because I can get Heavy Armour Proficiency + other things but I like the Knowledge Domain for the skills and I think Heavy Armour would impact my casting anyway so happy with the Medium Armour proficiency instead.

If I do this when is best? Do I start as a Cleric, pick it before level 3 or wait until after level x?

I am looking for the (hidden?) downsides of taking it at any given level (Or taking it at all given the party make-up), and hopefully I think the main downsides of multi-classing in general are quite limited in this instance. I will lose a pretty poor capstone, get some spells a little later and not much else.

My thinking is to pick it for level 2 (Fits thematically as well - criminal bard, reformed / repented but needed the freedom again so went back to being a bard without the criminal part), but as this is my first character and our campaign is planned to level 20 I want to get it right, and having a Strength score of 9 already hurts the inner min/maxer in me.

There have been a few questions for information which I deliberately kept out at the start so we can keep the focus. I wasn't looking for alternate builds as I thought that might be too broad but I do appreciate them so will give a bit more info.

Race: Half-Elf
Str: 9
Dex: 14 + 1 (Racial) = 15
Cha: 15 + 2 (Racial) = 17
Other stats: 13, 12, 10

If I am picking Cleric I have to put 13 into Wis, which then gives 12 Con and 10 Int. Straight Bard would be 13 Con, 12 Int and 10 Wis.

The effectiveness I am looking for is actually anything. I have looked at Warlock for the combat, Cleric for either the skills or proficiency and Rogue because it fits my concept. I am not fixed on Cleric and my main concern is that Bard 19/Something 1 is better overall than Bard 20 with no huge traps that I am going to fall into at certain levels (Being a level behind for key features I can live with since Lore already gets Magical Secrets early).

I am also allowed to use any officially published classes / books etc as long as I can show the GM where things came from. But I only own the DMG and PHB (Not yet sure what the GM owns) and would prefer not to have to Google every time I use a class feature.

Apologies to those who feel I have been misleading. I tried to keep the initial question succinct but probably took it off topic myself. I have accepted an answer which not only answers the initial question (IE: Multi-class early) but provided an alternate build which I am actually going to follow.