I imagine this being a bar chart with dimensions being (My Client #, the average other client)
"My Client #" would show a sum of the transaction amounts.
The "average other client" would display an average of the sums of the other two clients.
Post some sample data and I'll give it a go.
You should be able to do this by creating a couple of calculated fields.
The first calculated field would be find just the value for your client.
if [Client]="Client1" then [Measure] end
The second calculated field would find the value for the other two clients:
if [Client]="Client2" or [Client]="Client3" then [Measure] end
Then, you can place these on the view together side by side.
The Client field would not need to be on the view.
Hope this helps!