3 year old lying

My 3 year old is very smart and very independent. They're starting to lie now, and seemingly in a very mature way.

This morning they went to feed the cat (as they do every morning) and they made a big mess of cat food all over the floor. I asked them to clean it up (as they always clean all of their own messes and always have) and they came back awhile later saying they did all of it. I'm used to them telling the truth so I just said "good job, thank you for cleaning that up" and continued on with what I was doing.

When I finally went to check, I noticed they had hardly done any of it at all. I brought them in, explained that they lied to me, explained what lying was (they already know but it's a good reminder) and again went about my day.

They said once again that they did all of it. This time I asked them did they really do it, and they said yes. I went into the kitchen with them and they had cleaned half of it but definitely not all of it. I explained that they lied to me and put them in time out.

Later on they were working on a page in their workbook (we're homeschooling, this is something they knew how to do on their own and I needed to get something done really quick, they're usually not left alone during this) and they came in and said they had done all of it. I knew it hadn't been long enough and asked them thoroughly if they really did all of it, they said yes several times, I told them if they didn't really do it they're going to go in time out. I gave them lots of chances to change their answer. I went in, saw they hadn't done all of it and put them in time out.

What more can I be doing to teach them not to lie? 3 times in one day is ridiculous!