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Your best bet would be to not add any additional load to your existing 4" square steel posts.
I would add an additional 4" square steel post on the insides of two adjacent existing posts, this would make the new posts approx. 19.67' apart (center to center). Then I would size a steel beam that could rest on top of the two new shorter posts and be able to handle a 3 ton point load at the beams midspan. The new beam would have to be welded to the top of the new posts and the two new posts need to have an addition footing underneath then and they should be attached to the existing steel posts with mechanical connections.
I can help you size something like this for planning and budgeting purposes.
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