Not sure if I interpret correctly,
A) you'll be adding 2 more, which means eventually there'll be 3 X 3ft tanks? If so, then the total volume is about 170 X 3 = 510litres; chiller will probably need to be upsized to be about 1/4hp. - One option that I believe may work is: One 2026/28 for each tank (total 3 for this case), only 2 sets will drive chillers (ie. one for existing and another for your new chiller), the 3rd canister purely for filtering and water circulation only. All filter inlets draws water from bottom tank and outlet to individual tanks. 3rd level overflow to 2nd level and 2nd level overflow to lowest tank. Advantage of this setup is even is one canister / chiller fail, system will still be temporarily operational; albeit a higher load for the lone chiller (esp if left with the 1/10hp one, won't sustain too long).
B) New setup only add one more, so is still 2 X 3ft. - Similarly, one canister for each tank. Options may be either upsize chiller and use any one of the canister to drive; other canister purely for circulation only. Or add another chiller to be driven by the 2nd canister 2 chillers w 2 canisters) - added advantage is as mentioned in (A).
C) Regardless whether 2 or 3 tanks, resize all necessay equipment and upsize 1 big set for all tanks. I suspect it may be more costly, furthermore, with only one big canister operating, even in-flow / outflow distribution to the different tanks will need balancing. No contigency at all; one fail all fail.
Personally I see advantage in "casade" setups in terms of contingency and ease of maintenance in the long run. Have not consider cost implications, you may like to compare for yourself.
Just sharing some personal opinion, if I have made any wrong assumptions or interpret wrongly.... please pardon me.
Cheers

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