House training my 3 month old Lab puppy...HELP!?

My 3 month old Lab puppy is having multiple accidents in the house. I take her outside constantly and about 15 minutes after each meal and drinks. Her food is not down all day only at breakfast & dinner time. She will go outside and then come right back in and go on the carpet or floor within minutes after she has already gone pee & poo outside. What can I do? I am at my wits end and need ANY advice possible. I love her dearly...TIA