There must be a three for the price of two offer at Mercedes' design studios this month, because our spy photographers have caught a facelifted CLS, SL and SLK all testing together.The CLS, which has been on sale since 2004, gets a new grille in the style of the CL's, while at the back there are new tail lights and a new bumper. The changes won't be massive – Merc's four-door coupe has won a lot of fans for its smooth styling – but they will keep it looking fresh. Expect the tweaked car to go on sale in 2008.Next up is the SLK roadster. This is getting a slightly toned-down front, with a less prominent 'nose', but the addition of triangular bars underneath, mimicking those on Merc's F1 car. That means all the front air intakes are likely to change shape. The rear will also get a subtle update, including different lights.The driver of this test car was pretty keen to keep the dash out of view, so we can also expect to see some changes around the instrument binnacle, and also to the wheel design.Last up is the SL. We've already seen a modified SL undergoing testing, and this one seems to confirm what we'd first seen, namely that Merc is intent on refreshing its big roadster to help it keep going until the new model arrives in 2013.The SL was facelifted subtly in 2006, but this time it's likely to get much more substantial modifications. The two-seat SL will get a more radical CL/CLS-style front end: lights, grilles and the bumper are all being changed. As can be seen from the disguised rear, that's also in line for a revamp, including the lights, bumpers and tailpipes. The biggest change to the SL will be the replacement of the supercharged 5.4-litre V8 in the SL55 AMG for Merc's newer naturally aspirated 6.2-litre V8.