Getting rid of champions that you're not using (and have no future use for), or at least removing equipment from them, would help you reduce your player power and give you easier opponents. You might also want to look into getting a speed-lead for your team, High Khatun for example, and try to get her as fast as possible so that you get to be the one to go first. If you can go first with HK, then Serris to debuff and remove shields, followed by 2 nukers (or 1 nuker and 1 attack buffer if you have it), you should be able to get a bit higher at least. Unfortunately, there's no guarantee how high, because the matchmaking ensures that it's less about your team and more about your total power.

Your team is to slow. No speed-aura, no turnmeter-boost. Most opponent teams will go first and nuke you, before you can act. A champ like High Khatun could help here.

What equip do you use on your champs? If you go first and still can't bring them down with 3 damagedealers, something seems to be wrong with your artifacts.

The standard composition is to have a speed aura lead such as high katun to turn the meters of the other champs (kitted out in all speed gear), then have the second quickest be a debuffer to drop defence down and then nuke away- obviously you need the speeds to be appropiate so that they go in the correct order. Some will use two turn meter champs such as high katun and apothecary to allow their debuffer and nuker to be slow and in  gear optimised for damage/accuracy

How quick you need to be depends on player power more than your actual tier due to matchmaking

If your player power is low (under 400k for example) then you dont need to be too quick, maybe 200 speed for your speed lead and over 170 for the others.

If you are a mature account with high player power then to get out of bronze you will need far more speed as you will be matched against high power accounts, you would probably need 275+ speed as a minimum on the speed lead and 220+ on the others unless double turn metering or unless you have a 30% turn meter turner 

So you have the best debuffer going in my opinion

Go HK, then madame serries (get her to 60), then the two nukers 

Ok, I do have HK. Just not levelled her up yet. 

WIll try focussing on her next then.

On the subject of Nukers.

Do you think it would be better to use the two Kaels, or Kael and Soulbond Bowyer?

I dont have bowyer, looking at her kit she is excellent to take out one enemy (her infused arrow with ignore 75% def hit looks really strong)  whereas kael has a better aoe.

Try it out, i would prob want two big aoes, but - try both

You need a TM booster for sure. Decrease Def from Madame Serris+ a nuke from either Kael/Soulbound should be fine. You don't need another Kael for a 3rd nuker IMO (Maybe just gear the other one purely for CB). Also, what other champs do you have? You may be missing out on a key arena champ there. 

What is a TM booster, sorry?

TurnMeter booster, like High Khatun.