You pretty much need to use the API to trigger the other messages if you want the other messages to be delivered as soon as possible.

The flow could be like this:

• somebody opts-in
• in your response message (the 1st SMS) you put the content of the first message with added Ampscriptneeds to do the following to trigger the other 2 messages:
  • get an API token
  • either trigger a journey that is sending the other two SMS or make separate API calls to different API-triggered SMS from Mobile Connect

You could use the script developed by Ivan Razine and just adjust it to send SMS instead of emails: How to trigger a Journey API Entry event with AMPscript and server-side JavaScript

This might result in some delay between the first two messages or even cause the 1st message to be delivered later than the 2nd or 3rd message. This risk exists because execution of the API call in Ampscript delays the send of the 1st message a bit.

To ensure the messages get sent in order, you would need to use a trick and effectively set up 4 messages.

This time the mobile opt-in response SMS wouldn't send the content - it would just serve as a way of injecting the customer into the API-triggered journey during the send preparation phase.

You would need to force the message to be canceled, which you can do by ensuring that there is nothing sent to the customer (not a single letter or character) - empty SMS sends are not performed in the end.

Check out this example script in an SMS:

%%[ 
/* API injection code */ 
]%%

This message will be canceled unless you use the Output function inside of the script or add anything outside of the script block. RaiseError might also work as a canceling method, but I don't remember if it works correctly in this context

In the journey you include all 3 of your SMS and so you shouldn't have to worry about the message send order anymore.

You pretty much need to use the API to trigger the other messages if you want them to be delivered as soon as possible.

The flow could be like this:

• somebody opts-in
• in your response message (the 1st SMS) you put the content of the first message with added Ampscriptneeds to do the following to trigger the other 2 messages:
  • get an API token
  • either trigger a journey that is sending the other two SMS or make separate API calls to different API-triggered SMS from Mobile Connect

You could use the script developed by Ivan Razine and just adjust it to send SMS instead of emails: How to trigger a Journey API Entry event with AMPscript and server-side JavaScript

This might result in some delay between the first two messages or even cause the 1st message to be delivered later than the 2nd or 3rd message. This risk exists because execution of the API call in Ampscript delays the send of the 1st message a bit.

To ensure the messages get sent in order, you would need to use a trick and effectively set up 4 messages.

This time the mobile opt-in response SMS wouldn't send the content - it would just serve as a way of injecting the customer into the API-triggered journey during the send preparation phase.

You would need to force the message to be canceled, which you can do by ensuring that there is nothing sent to the customer (not a single letter or character) - empty SMS sends are not performed in the end.

Check out this example script in an SMS:

%%[ 
/* API injection code */ 
]%%

This message will be canceled unless you use the Output function inside of the script or add anything outside of the script block. RaiseError might also work as a canceling method, but I don't remember if it works correctly in this context

In the journey you include all 3 of your SMS and so you shouldn't have to worry about the message send order anymore.