i'm sorry for my unclear word
.
I don't know your circuit, so i will assume those 3 having same source. So your arrange will be somehow like:
S|0.7u|D D|1u|S D|0.7u|S (S/D are source/drain)
so my idea will be:
S|dummy|D|0.7u|S|0.5u|D|0.5u|S|0.7u|D|dummy|S
I will split 1u width, 1 finger transitor to 0.5u width, 2 fingers transistor, sharing source for our 3 active transistors.
About the dummy, actually, adding them will help you layout result closer to circuit result. As you know, thing on silicon will be different with your layout, in this case, Vth of 0.7u transistor will be different with your design due to systematic factors such as STI stress,... or due to etching step in fabrication. So adding side dummy will help you minimize these effects. If you worry about coupling, you can see there is no different of Cdb or Cdg of 0.7u transistor.
About connection of dummy, since transistor is operated by voltage, you just tie its gate to vdd (if pmos) or vss (if nmos) for deactivate it (just like how inverter works). Sharing terminal of dummy with your active transistor for eliminating STI area at the side of active transistor.
About "fill middle deadspace made 0.5u by diffusion layer", as you can see in my placement, there will be a little STI deadspace above 0.5u, 2 finger transistor. You can reduce unexpected effect caused by this area by fill in diffusion layer (the one create source/drain), connect it with vdd/vss (depend on your bulk connection).
about the different dimensions thing, honestly, I've not seen any matching part with different dimension up till now. And in my opinion, you can't get devices having matched characteristics when they have different dimensions if you applied systematic effects or other effects in real silicon. Even some foundry like TSMC or Samsung recommend use same dimensions for matching devices in their design manual.
I think you should study on cmos systematic effect, and random effects... (might be a little cmos fabrication for more understanding why these effects happen), then apply on you layout, you won't be confuse anymore.