SLA (Resin) is rarely solid, not just for monetary reasons but also technical reasons and need to be designed for it. SLS will be solid unless you leave holes to bleed powder from the cavities and even then you will likely still be charged for it so you may as well make it solid.
3mm is a pretty solid piece for something that has sides for support such as a 60% case, but for a flat part, 3mm is not that tough. Remember, even sintered, these are not going to be as dense as injection molded parts and it's still plastic. I did my plate at 5mm total thickness, maybe see what the price difference is and go from there, if it's not much more, go thicker. You only need to leave the bottom of the switch available in order to solder, so why not use as much as you can?
0.15mm accuracy is quite good, but I would expect nothing less from a $150k+ SLS machine (which is what Shapeways uses).
FDM industry average is 1mm or 1% whichever is larger, though 3dhubs is trying to cut that in half, many machines, especially deltas can struggle to even make the old standard.
Keep in mind, accuracy is not the same as tolerances.
Accuracy tends to be about overall dimensions, while tolerances tend to be about clearance. You can have a 200mm plate that accurate to 0.15mm which is awesome, but if you make your bolt holes with that as a tolerance the bolts may not slip through your holes. If you make your plate a press fit with that as a tolerance, will it slip together or need some sanding? See the difference?
This is why I said, have a small sample done to check your clearances, you want a slip fit or even a slightly loose fit for switches. Get it too tight and you are either ordering another revision (at full price) or spending a lot of hours with a jewelers file. If you are not sure and just want to go for it, give each hole 0.2mm clearance per switch side, so .4mm across and .4mm vertically. That should be enough regardless and still be tight enough for the switch to grab. If you do a test, it doesn't even need the full skirt, just the switch hole and some material around it (say 5mm).