What does your Uni Suggest?
SKTEEP, check with the Engineering department and find out what they recommend. You will want to buy what your professors and classmates use. If the department has no recommendations then make sure you go kick the tires of all three and think about the cost of the applications you will need to buy as well like office not just the sticker price. In addition, check battery life and weight; the Lenovo and Dell will have good battery life. Not sure how good Mac battery life is. I use a Lenovo for work and own a Dell XPS for personal use, both OEMs make good notebooks. Hope some of this helps.
Disclosure: TimWoodChip is an Intel employee and aspirational sponsor of tomorrow.