We got an egg today that weighed 3.30 oz, it wont even fit in the egg carton. We average around 1.80-2.0 oz per egg, except for the hamburgs which are around 1.25oz.
What you have is almost certainly a double yolked egg. Sometimes, when the hen's internal egg factory messes up, she'll start two yolks at the same time. The result is usually an extremely large egg.
It usually does not harm the hen. On rare occasions it can cause prolapse or possibly other problems, but not really very often. This will sometimes happen when a pullet has first started laying and does not have alll the kinks worked out of her internal laying system, but it can happen with older hens too. As long as it is not a regular occurrence, it usually is not a real problem, but I'd prefer to not see this be a habit for a hen. There is some risk no matter how neat or cool it may be to get an egg like this.
Same thing happened to us. Have 3 layers and were getting consistent 2" eggs, then this one. All I can say is OUCH! Haven't had another since and didn't seem to effect the one who laid it as she went back to the regular size ever since.