In English (apologies my German is not the best):
Note:
* Philips Hue Bridge takes care of much/most of the communication with Apple Homekit. Hence any lights, lamps will be typically automatically exposed and controllable from the Home App.
* In more complex setups - where you combine multiple non-Philips (but Zigbee and compatible) lights as well as scenes, dynamic lights etc you may fall into the rabbit hole of Homebridge / Node-Red etc to run more advanced automations etc.
* The Shelly script in theory simply calls a basic API request to the Hue Bridge ... so most lighting functions are "easily" triggered.
To your specific questions:
* Yes it's possible. That is the setup I currently have.
* Yes - best to create a 'group' in the Hue app directly off those 3 lightbulbs. That way your script only needs to 'turn on' or set color of the group. (Hue Bridge API is limited in number of calls you can do, so most efficient to group them)
* You will need a shelly at every point that you need to capture the state change / toggle based on how you want to use. Depends on the wiring in your house.
e.g. my stairs lights are controlled with 3 switches (bottom, middle, top) that are traditionally wired. I have a single shelly 'sense' the change in on/off at the end-point, i.e. where the wire goes to the light bulb. That way I have 3 'dumb' switches, traditional wiring, and only 1 shelly to monitor on/off/toggle.
There are other solutions
1. Using Zigbee / Hue or Friends of Hue light switches. Especially if you expect to go the Zigbee/Hue route these are a simple way to setup - can be placed over the original switch locations. (I personally do not like the aesthetic)
2. Philips Hue also has their own counterpart to the i4 which will be directly compatible with Hue Bridge - and hence also into HomeKit.