I took claritin 3 hours ago, for mild shrimp allergy. can i take benadryl because claritin didn't work?

Yes : If you have just started claritin it does take a good 18 hrs or more to get up to effective levels. Benadryl gets there in an hour & can bind with histamine receptors more firmly. I would consider it a better drug for your use for that reason & it will not interfere in any important way with the claritin. The Benadryl is more likely to sedate you while used in usual doses.