Yes, open primaries help third parties as otherwise, those parties may never even have the chance to participate in the primaries in the first place. If a party cannot even participate in a primary, then the party is certainly not being serviced as the part cannot get its message and candidates out to the populace.
An open primary allows someone to vote for any canidate they choose. They do not have to be affiliated with that party. This is good for third parties because someone may be registered democrat or republican and then decide they really like a Green Party candidate. An open primary would allow them to vote for that person.