Assuming you have the milling attachment for the U3 then it should cut 3mm brass OK, mine has done a lot worse in the past. If you saw to within 0.5mm of the line then it's only a cleanup pass or two at the full 3mm height. Looking at the engine it is only the bottom edge that really needs to be true, the rest could just as easily be sawn and then filed to shape.
One issue you are likely to get with water jet cutting is a slight taper to the cut face which will mean your plates don't stand up vertically unless you can find one with a machine that tilts the head to compensate for this. Also most cutters charge by machine time so you pay more for a fine finish than a coarse one. If you did want to get a price then send a .DXF file to Sciss for a quote though their site is not working a sI write this.