1) The Bird Dog: the bird Dog is en exercise that strengthens the core; in other words, it strengthens the abdominal muscles, lower back, butt and thighs. 

The bird dog is a bodyweight exercise, which means you need no equipment because your body provides the resistance. The main target of this exercise is what's called the erector-spinae muscle. It is the muscle that extends, flexes, and roates the spine. The bird dog also engages your gluteus maximus muscle, the trapezius muscles of the upper back and the deltoids of your shoulder.  

Find step-by-step instructions for this exercise here. 

2) The Dead Bug: the dead bug engages your deep inner core, which includes your transverse abdominus and pelvic floor. 

Here are step-by-step instructions for this exercise. 

3) Low side plank

Here are step-by-step instructions for this exercise, as well as variations that can make the exercise more challenging.