Jessica Simpson says having three kids is ''no joke''.

The 39-year-old singer - who has Birdie, three months, Ace, six, and Maxwell, seven, with her husband Eric Johnson - admits it is a ''big challenge'' sharing her time between her three children.

She told People magazine: ''Three kids is no joke. It is definitely constant, and the biggest challenge for me is trying to be present when I am pulled in so many directions. They are all in such different phases now.''

Jessica previously revealed she is ''working really hard'' on her post-baby body.

The 39-year-old singer said: ''I am working really hard right now. It's not easy at all, but I am determined to feel good. I have been doing a lot of walking - getting my steps in not only burns calories but it also helps me clear my head and get focused. I am eating healthy too - I discovered I really like cauliflower. Who knew it could be a substitute for almost anything?''

Jessica confessed she ''doesn't understand'' how she got pregnant again.

She admitted: ''I thought about [having another baby], but I didn't really know that it could actually happen. We definitely always love to practice. We actually weren't practicing very much that month so I don't understand. We try [to figure it out]. This might be a miracle baby.''

But her children were ''so excited'' to be a big brother and big sister.

She added: ''They're so excited because they're only fourteen months apart, so they didn't get to experience the excitement of each other. But when I told them that we're having another baby, I know that they'll remember that for the rest of their lives.''