To see P&L per trade, add the "Realized P&L" field to the Trades window (in Mosaic, it is a tab on the Activity panel; in Classic, click Trade Log from the Account menu). This field shows the total P&L on the trade, and if you use the "+" sign to view each fill you will see the P&L for each fill on that trade.
You can also review the Trade section of your Activity Statement to see the realized P&L for each trade. Access the Activity Statement from the Performance & Reports menu by selecting Statements and clicking "RUN STATEMENT" for the date period you have selected.
Find out more about the TWS trade log in the TWS Users' Guide.