Apologies if you thought I'd be offering some creative hack for tying laces in the 3rd trimester, I don't really have any. My only advice would be to either
A. Cross your leg over your knee so your ankle is resting on your thigh to tie them. This can be uncomfortable if you have pelvic Girdle pain.
Or
B. Don't wear shoes with laces. If you're anything like me, your feet may be too swollen to wear anything other than flip flops or sliders when in the late stages of pregnancy.