There is a comma splice in the second sentence, but apart from that it is grammatically correct. You should replace the comma by a full stop as there are two sentences here, or use a conjunction
It may be better to use a more precise verb than "had" in the first sentence "I have owned my camera for 3 weeks."
There is a question of meaning. The first sentence is a present tense sentence. It speaks of currently having a camera, now and in the past three weeks. This is the meaning of the present perfect. But the second sentence contradicts this. The grammar is fine, the meaning is contradictory.
There are other uses of the present perfect. But since you talk about "my camera", the first sentence must be referring to a camera you still have. Other uses include "present experience", for example "I have climbed Mount Fuji but I didn't enjoy it." The first part tell of an experience that we have (present perfect) the second part tells of the past event that gave us that experience. No grammar error, and this time no contradiction.
I don't know what the first sentence is supposed to mean. It looks incomplete.