webMethods.io Integration FlowServices do not yet support regular expressions.
As an alternate solution you could use the Substring utility to extract the first three characters from the string, and then use IF statements to determine if the substring value is equal.
Obviously the begin and end index used in the Substring can be whatever you need (e.g. make these dynamic).
In the first step, the Substring is checking String1, using a beginindex of 0 and endIndex of 3.
The subStringValue is the first three characters of the string.
The rest of the code uses regular IF statements to check if subStringValue equals to the compare value (e.g. ABC, DEF, or GHI).
In the on-premise Flow Services world you can use regular expressions.
Do you always want to check which characters are at the beginning? Of course you can make the begin index and end index used in the Substring dynamic using parameters.
Anyway, this is a quick and dirty way. I am checking with our R&D team about when wM.io Integration FlowServices will support regular expressons.