Create new grouping column which is combination of Group and Subgroup to make it a bit easier. Then write a formula which will get values for specific group for LogV and LogT3 (many options for this). After this you can use Linear Regression function to get the estimates for slope. If you really want to see only one value for each group, then add check that if you are on first row of the group -> perform calculations, otherwise return missing.
-Jarmo