Assuming that I remember the FM-3 schematic properly, the eye tube is solely used as an indicator. One beam (lower) is for tuning and the upper one for stereo indication. You can always leave a "dead eye" tube in place. It just won't look quite as cool.
I normally tune "eyeless" FM receivers by ear. The high frequencies get clearest at proper tuning. Expect some minor drift as the unit reaches normal operating temperature.
Info here: http://www.triodeel.com/schindex.htm