Hi there.
Indeed, you will be able to route the SMS to the RG/Queue of the agents, or to a specific extension directly too.
So what you will need to do in order to receive the SMS, is make sure that the number that is used for the SMS is assign to RG/Queue/extension so that the 3CX system can route it correctly. In the event if the SMS arrives to the RG/Queue, all the agents will receive a notification in the 3CX app, just like if it was a message from the live chart facebook/website integration (idea is the same).
When it comes to send the SMS, since you will be a memeber of the RG/Queue you will be able to send it out, and if you are a memeber of multiple Ring groups/Queues, you will be able to choose from which number to send the message out.