I've asked Gemini about it the other day(I'm dumb and shameless). Apparently it means that the model branches into bunch of 3B sections in the middle and joins at both ends, totaling in parameters at 30B. This means computational footprint reduces to (bottom "router" parts + 3B + top parts) of effectively-5B or whatever specific to that model implied by "3B", rather than the full 30B.

MoE models still operate on token-by-token basis, i.e. "pot/at/o" -> "12345/7654/8472". "Experts" are selected on per-token basis, not per-interation, so "expert" naming might be a bit of a misnomer, or marketing.