Not sure just what you mean by the 'pre-interview panel'. Do you mean the application and the supporters' statements have been sent to the Assessors for review and you are waiting to hear if you will be going forward to interview, or do you mean that you have been told that you will be put forward for interview and are waiting for a date? Either way, three weeks is a fairly short time, since you should remember that the people involved are all volunteers so this is not a full time job for them.

In the current climate there are delays over and above normal, though the IET are doing their best to minimise these. I wrote about these delays at https://communities.theiet.org/discussions/viewtopic/795/26458 at the beginning of last month, so will not repeat it here. Information about the length of time normally expected for applications to be processed is available under the FAQ on the IET website, but I don't think that has been updated to account for COVID-19 problems.

Hope this helps.

Alasdair