You could approach this two ways:
1. Iterate through every 'sentence' and check the value of the first three characters. eg.
Code:
Sub aTest()
Dim aSentence As Range
For Each aSentence In ActiveDocument.Sentences
If Left(aSentence.Text, 3) = "com" Then
'Do something
Debug.Print aSentence.Text
End If
Next aSentence
End Sub
2. Do a find in the document for instances of "com" and then check if that appears at the start of a sentence.
#1 uses Word's definition of a sentence so it is simpler but is likely to be slower.
#2 would probably be faster but you would need to work out a definition of what constitutes a Sentence.