My pulse had a third beat. is this an irregular heartbeat?

Maybe: When you listen to your heart, you typically hear a "lub-dub, lub-dub" sound. If you hear a "lub-dub, thud" on occasion, this might be an extra beat, usually a premature beat from the upper chamber of the heart (may less often be from lower chamber, PVC). This can be clarified with a heart monitor. Check with your doctor to determine if this monitoring is necessary.