There is indeed a problem - but it's probably not the one you're thinking of.

I'm guessing you are thinking that adding Geomancer in doesn't work well because his HP Burn gets replaced/removed by Sicia and/or Ninja. And while that is true, it's not the reason why he won't work well on the team.

The real reason is that you will almost never be able to afford to run three people whose only job on the team is damage. Typically, your team needs to have:

1. Someone to handle Decay. This is non-negotiable. If you don't have someone on the team with provoke, either directly or by virtue of using Taunting sets, Decay will quite simply end your run. And even with Taunting sets, you'll almost certainly need *someone* with an actual provoke, just to be there in case the set bonus doesn't proc. So, that's one spot - let's say for the sake of argument you use someone like Seeker/Sandlashed.
2. Someone to handle Mischief. While there is a slightly greater chance you could get by without this, I would be extremely doubtful if the costs would warrant it. Generally Mischief is handled either by someone with Block Buffs, or by someone who can be guaranteed to have the most buffs and just run high resist. Unfortunately, of the champs on the team so far, none can do that latter part. Ninja does at times have the benefit of Perfect Veil, but relying on that to ensure he has the most buffs is not going to be easy. So, let's say we put someone in here to handle that - we'll run Ukko for this, since he also brings Speed Up.
3. The other four heads don't typically require someone exclusively to handle them, so that leaves us with one spot left on the team. However, so far, we don't have anyone at all to keep the team alive. We also don't have anyone with Speed Down debuffs, which are super, super important. And unfortunately, as far as I know, there is nobody who can do both of those things.

That's why you can't run Sicia, Ninja, and Geo on the same team. There just isn't room. In our aforementioned team, if we use only two of those three, then we can put, say, Mithrala for the team survival and Tainix Hateflower for the healing and speed down. And voilà - we have a very solid team composed of champs that you almost certainly have:

1. Ninja (free)
2. Geomancer (epic, therefore "relatively" common, and you already indicated you have him) 
3. Ukko (recent fusion champ)
4. Mithrala (free)
5. Tainix (free)
6. Seeker/Sandlashed (epic, therefore "relatively" common)

well i put my duchess for sure...sicia for defense down....geo for accaracy down damage....have nekhret but dont think it will work...

yeah i need uukko

thanks man

There are quite a few options for AoE block buffs other than Ukko, though a lot are legendary:

Pinthroy, Deliana, (sort of) Kimi, Kantra and Norog come to mind. And of course there's Uugo at Epic.

With regards to your question you could make it work by having SIcia go first but the issues will come along with her A2 interacting with Geo's A3, I'd think. Personally I like the idea of keeping your Sicia/Ninja together in one team, and Geo in another if you can. 

What difficulty/difficulties are you hitting?

neverming i just go for hydra 1....just feels bad to have sicia ninja geo...also have mordecai...

but not be able to use all same time yeah

Why do you want to use them at the same time when you have to build 3 Hydra teams anyway and champions used once are blocked for the rest of the week?

if you cant make 3 average teams but can make 1 key team

= make 1 key team

Actually for hydra, u are probably better having 3 average teams and getting one chest from 3 levels...

Either way using those 3 on 1 team is a terrible idea.

have to agree here with trying to avoid frontloading a team too much

Depending on what OP's roster/gearing looks like would rather run something like Mordecai/Ninja (or a more random comp, move Ninja up to Hard with Sicia) at Normal, Sicia Hard and Geo Brutal, something like that. Normal I think we can just turbo smash and not worry as much about interacting profitably with Decay, if it pops up