First off, kudos for taking the initiative! Running Ethernet is always the best option when possible. A few things to consider:

1. Make sure the second router is positioned optimally - central and away from obstructions.

2. If the third floor extender is dual-band, try using the 5GHz band for the wireless backhaul to the second router. Less range but faster speeds and less interference.

3. Play with the extender's antenna positioning to find the sweet spot for reception.

Hope this helps!