If a Catholic seeking to covert to Orthoodxy was baptized by tripple sprinlking rather than tripple immersion/pouring, or of the Orthodox bishop has reason to believe that sprinkling was used instead of tripple immersion/pouring, the individual would be baptized, not re-baptized.
It's like someone whose marriage was annulled by the Church. That person was never married. In this case, as far as the Orthodox Church is concerned, that Catholic was never baptized.
Needless to say, baptims of Catholics is rare nowadays. Protestants who are usually baptized with a single immersion/pouring are routinely baptized into Orthodoxy.
But all converts to Orthodoxy, save for the Coptics, Ethiopians, Armenians, etc., would be chrismated (confirmed) with holy oil, regardless if they require baptism or not.
Catholic infant baptisms usually involve water being poured over the head three times, so I guess that would be valid.