That picture does not look real, but that's not the point.
The Christian belief is that God created Adam and Eve with two legs, not three. God created Adam and Eve with the ability to procreate babies with two legs, not three. Because of the sin of Adam, harmful mutations and birth defects entered this world, hence three legged babies.
Also, regardless of the sin of Adam, certain individuals make their own bad choices, personal sins, such as the ones Beach Bum already pointed out, which affect their unborn babies, hence the three legged baby.
Even if you do not believe in God, you could say the same about nature. Humans do not have three legs, normally. If a baby is born with three legs it is understood that this would be a birth defect, a harmful mutation, or consequences of the parents bad choices. But it is accepted that naturally, this isn't the norm.
Birth defects and harmful mutations are not a good argument against intelligent design. If anything, harmful mutations have been used as an argument against evolution, which depends on beneficial/positive mutations.