The paragraph and font formatting applied to envelope addresses are determined by the Envelope Address style of the current template (or of the current document, if it's different from that of the template).
The default line spacing in that is Single, so if yours is triple or some other large spacing, it's been changed somehow.
To restore single spacing in the simplest way:
Open the Envelopes dialog, enter any text in the address box, and click Add To Document.
Click inside the address text of the envelope that was inserted.
If you have Word 2003 or earlier, click Format > Styles & Formatting. If you have Word 2007 or later, press Ctrl+Alt+Shift+S. Either way, you get the Styles & Formatting pane on the right, with the Envelope Address style highlighted.
Right-click the Envelope Address style in the pane, and choose Modify.
In the Formatting section of the Modify Style dialog, there are three buttons for Single, 1.5 Line, and Double spacing. Click the Single Space button.
In Word 2003, check the Add to Template box; in Word 2007 or later, click the "New documents based on this template" option.
As verification, the address in the inserted envelope should immediately appear single-spaced.
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