In my experience, PD does a really good job of matching content even across widely varying audio levels, clarity and ambient/background noise. Sometimes though, the 3rd or 4th clips may cause conflict as PD can't sync all of them together.
I suggest you try syncing the audio track and one of the video clips outside of the MCD. Place both clips on the main timeline, then select both and click on Sync by Audio.
If you can get those two in sync, select both clips again (if they aren't already selected) and move them to the start of the timeline, so at least one is touching the left edge. If the audio track starts first, you're ready for the next step. If the video clip starts first, place a timeline marker at the exact spot where the audio clip starts. Now save the project.
Next you can try to add the second video clip to the timeline, then select that clip and the audio track and use Sync by Audio again. These two should now be synced, and you can drag them up and to the left and drop them in place with the audio track exactly at the start or aligned with the marker. Save the project again.
Now you have a choice. Since you already have everything synced up on the main timeline, you can actually do all your work there, simply cutting or dropping the opacity to 0 on the lower video clip everywhere you want the upper clip's content to be shown.
If you want to use the MCD, the best solution might be to trim the leading edges of each clip so they all start at the exact same time, and then disable two of the timeline tracks and produce each clip separately. When you're finished, you can simply import the new clips into the MCD and not worry about syncing.
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