Yes, You can. I am using it that way.

U already answer ur own questions.
GOOD JOB.

What else u need to know here?

if i not mistaken , xiaomi router 3 repeats at 2.4Ghz only , which means it can't go more than 100Mbps

Yes, but actual throughput is bad.

I have use repeater mode in my pass when I'm on 100mbps and I have no problem getting ~100mbps on WiFi via mi router.

The WiFi linking speed between router can be 866mbps if very close (~5m).
Then my phone device also can link to mi router at 866mbps.

In actual use here, I will make assumptions here the cpu of 800mhz can only handle upto 300mbps from WiFi.
Divide by 2 due repeater cut into half efficient.
My guess is max out at 150mbps throughput.

If I could find time on this weekend, I will try it out for u. No promise.

Tested.

Mi Router 3 as repeater (AP client +AP) using 5g.

On LAN, speedtest around 90 mbps DL & UL.
Then on my wifi using my P20pro, it reach around 15x~18x mbps DL & 13x mbps UL.

This is what I had expected earlier around 150mbps.