How cool would it be if we could flag a review that involves 3 or more participants, when submitting it, and subsequently be able to search for such type of reviews?

Do you mean three = Client + 2 Providers? ... or even +3 Providers = 4 total?
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Not everyone completes their duo (threesome) or trio (foursome) Reviews the same way, but they are supposed to include both (or all three) Providers in the name field as "Ann and Betty" or "Carol & Doris" or "Ellen and Freda & Giselle," and so on. Searching the name field for "&" should produce genuine duos and more since nobody has an "&" in their name. Searching the name field for "and" should produce a lot of actual duos and moresomes but also a lot of "ANDy," "brANDy," "cANDy," and so on.  
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Why is this on the NY board? It should be on Suggestion and Policy (as a suggestion) or Newbie (as a "How do I search for 3 or moresomes?" or General is a general comment.  
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But there is agreement that TER needs a better way to do 3-some and more-some Reviews. There is only one URL field, one phone number field, ... and only room for physical attributes of one unspecified hostess.

Thanks for replying, understood and agreed.

What you described is a subset of what I’m referring to.

Recently, prior to submitting a review of a MFM session with a Top 10 NYC provider and a buddy, I sent a “Contact us” note to TER, asking if I should submit a review under the provider’s existing profile or under a new profile, (E.g. Provider Name and Male Friend)

TER response was very clear: “You need to submit it under the existing profile, as Male Friend label is for the male escort, not a client.”

Meaning, the “ProviderA and ProviderB and …” profile name should be used only for a session with a client and two or more providers.

A session with a provider, a client and a non-provider (client’s significant other or a buddy/friend), should be submitted under the provider’s existing profile.

There is a decent number of such reviews, and unless one stumbles across such a review, no search function would list it.