Max volume at -3 db when adding any audio effect

To help narrow things down, what happens if you apply an effect to the track instead of the clip?

Yes, that's basically the situation.
The clip can peak at -1 or be a fully mastered music .mp3 peaking at 0. As soon as I add an audio effect, the ceiling becomes -3.

Just to make sure I understand correctly:

• You have a clip that peaks at -6 dBFS in the Source Monitor.
• After adding it to the timeline, you increase the Volume by +6 dB, and the Audio Meter shows 0 dBFS.
• But when you add any effect to the clip, the Audio Meter then shows -3 dBFS instead.

This happens to mono and stereo clips. They don't have the -3db ceiling until I add any audio effect except for "Volume". 

Are your clips mono? If you’re adding a mono clip at 0 dBFS to a Standard Track, Premiere Pro will convert it to stereo (panning equally to both channels). This causes the level to register at about -3 dBFS due to the panning law.

Huh ... that's just not behavior I'm familiar with. Maybe @PaulMurphy could advise?

I do wonder at the reason for going clear to 0?  As for everything I've done, they've wanted an absolute limit around -3dB at least, some at -6dB. As clipping occurs with anything at 0 and above.

It happens with all audio. The original can be super low, or it can be full on clipping. Whether it's raw audio from the footage, or even a fully mixed and mastered music track, the second I add any audio effect besides "Volume", the clipping point becomes -3db and I have to add an additional instance of "Volume" at the very bottom to add 3db to bring it back  up to 0.

Wow ... I've never had this happen in over a decade! I can just G to add gain, and push to 0 or past, and start clipping, with any clip with solid levels in the original recording.

What level does your original clip show in the meters?