Firstly, a flat tummy is made in the kitchen primarily. You have to watch what you eat to get a decent stomach. The exercises will just add the tone.
With the planks just try and add five seconds each time you do it.
Build up your reps with hanging knee raises. If you’re doing 5 reps do at least 3 sets with a short break in between each set. After that do one last set but just raise one knee at a time.
With the L hang hold the knees up position and extend your legs out to the 45 degree angle. Just work on holding it for time. When you get better you’ll be able to raise it higher.
You can get really toned abs using just one ab exercise but if you’re doing squats, pull ups and push ups as well then these are all additional ab exercises. Concentrate on engaging your abs and lower back in all of these exercises.