This is absolutely NOT ridiculous. If I were getting married and had a 3 yr. old daughter, I would definitely want her in my wedding. I would feel funny having other children in my wedding instead of my own. Having your daughter be the flower girl would be beautiful. I realize that some people feel the children participating in wedding ceremonies should be older, but this is different because it's your own daughter.
I had three flower girls in my wedding. One was 3 1/2 and the other two were only 2 yrs. old. Yes, I knew there was a chance that none would participate when the time came. I knew that, and I didn't let it bother me. When the ceremony came, one of the 2 yr. olds didn't want to walk down the aisle. Her mom was standing by, took her, and they both went and sat down. It didn't bother me, and everything worked out fine. Best wishes to you!