SIM card for Germany, Austria, and Swiss

[inquisitor]

Data roaming with Lidl would cost € 1.50/MB in Austria and € 12.00/MB in Switzerland. Given these rates a separate SIM for each country would quickly pay off.

If you activated the 500MB data pack with Lidl you would automatically benefit from the "EU Internet-Tages-Pack" which applies in EU countries (so not in Switzerland as it is no part of the EU) and gives you 10MB for € 3/day (afterwards you get disconnected until midnight).
Unfortunately this "EU Internet-Tages-Pack" is only available in combination with the 500MB-pack. If you activate any other data option (even the bigger 5GB pack) you will be stuck to the volume-based tariffs given above.