Can you provide the full CPU names? While I assume they're either 8250U or 8550U there are some minor differences that might be relevant for CAD work.
Will you be using the laptop on-the-go, or will you mainly be working from an office desk? Lenovo IdeaPads typically offer 4 to 5 hours of battery life, while ASUS laptops are often quite long-lasting - in the 9 to 10 hours range. Acers tend to fall somewhere in the middle, something like 6 to 7 hours.
In terms of build-quality, I would rate Lenovo ahead of ASUS with Acer in the last spot.
OK, it's the 1.6GHz version that will boost to 3.40GHz. I would imagine for light CAD projects this CPU/GPU setup should be adequate. At some point, depending on the specific software you're using and your career, you might consider a Xeon-based laptop.
I would still recommend the Lenovo, not least because of the Novo button that will allow you to reset/restore the computer if Windows decides to die on you. This feature has saved me on more occasions than I care to admit, and it's pretty much unique to Lenovo machines.