Use 3/32" electrodes and about 90-110 amps. check the other side of the plate to see if you have adequate penetration. If not, increase your current and/or slow down your weld speed. The main thing you worry about here is distortion, if it's a butt weld, or even with a 90 degree fillet weld. You minimize distortion by minimizing heat buildup in any one area. Use the "skip welding" technique. Tack weld the two plates in about four places. Make sure you make fairly solid tacks. Then make several short welds about 6-8cm long at maybe 4 places along the joint,with fairly even spacing. flip the assembly over and weld the opposite sides of the welds you just made, then make 5-6 more welds between the existing welds, and repeat this process until you completely filled in the joint. You'll end up flipping the plate over quite a few times. With only 3mm you can get away with welding only one side since it's possible to produce full penetration welds, But that introduces most of weld stresses on one side of the plate only which is sure to cause warping in that direction. So it's best to weld both sides.