Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you would like to know if there will be ExpressRoute Support in East US 3 location.
Currently, East US 3 location is yet to come.
And as such, no resources can be deployed in this region.
Now, You can use this site : Azure Regionsto check the available regions
Once the region becomes available, you can use Products available by region site to check the ExpressRoute support
Kindly let us know if this helps or you need further assistance on this issue.
Thanks,
Kapil