There continues to be debate over the optimal size for a small committee, but one common finding is that a group of three people often spells trouble. The most common problem is exactly what you are experiencing, namely two against one.
There can be any number of reasons behind your exclusion by the other two members. For example, perhaps they want all of the possible recognition or glory from this project, or maybe they believe that your inputs are not as valuable as you think. It is also possible that they feel threatened by you, and your expertise could make them look bad. Or, perhaps they just want to make you look bad.
Regardless, since you are a member of this committee, you should be an equal participant. The only reason this duo is able to operate without you is that you are letting them do so. You should meet with them and insist that you be a part of all meetings, discussions, and communications related to this committee.
If they continue to play two against one, you should get someone else on your side, such as the manager who formed the committee in the first place.
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