Kyiv is a huge city with lots to see. I was there early last month and I had slightly shorter time than you:
D1: Flight landed in Kyiv at about 2pm
D2: Chernobyl day tour
D3: Leave Kyiv at 9pm (flight)
Unfortunately I did not manage to cover all the sights I intended to
Some caveats on why I didn’t manage:
1. I was quite tired/lazy on day 1 (cause it’s towards the end of my trip) so all I did that day was walk around the Independence Square/Khreschatyk Street, dinner and a “metro station tour” on my own.
2. The Chernobyl tour took almost the whole day. We were back in Kyiv at about 7pm. Though there’s still few hours to explore the town but I decided to dine with a new friend I met during the tour and hence whole day gone.
3. I joined a walking tour on day 3 and that took up a few hours.
So if you are not lazy, didn’t make new friends and didn’t join walking tour, you may be able to explore more. But I’ll still recommend adding another day in Kyiv so as not to be too rush.
Minsk, on the other hand, can be covered in one full day.
Safe travels!