\/[0-9]{3,} will find only numbers >99 that are preceded by a \.

However, maybe you will need to put the numeric strings that you are
searching for in an array and then have the macro step through the array to
find each one in turn.

Here is an example using the three numeric strings that you mentioned:

Dim vFindText As Variant
Dim myrange As Range
Dim i As Long

vFindText = Array("/6346", "/137", "/138")
For i = LBound(vFindText) To UBound(vFindText)
   Selection.HomeKey wdStory
   Selection.Find.ClearFormatting
   With Selection.Find
       Do While .Execute(findText:=vFindText(i), Forward:=True,
MatchWildcards:=False, Wrap:=wdFindStop) = True
           Set myrange = Selection.Bookmarks("\line").Range
           With myrange.Font
               .Bold = wdToggle
               .Color = wdColorRed
           End With
           Selection.Collapse wdCollapseEnd
       Loop
   End With
Next i