New Hampshire border to Rte. 495
Rte. 495 to Treble Cove Rd
This can be bad in the mornings, though much better than in years past when Rte. 3 was 2 lanes wide. Rte. 495 empties two lanes onto Rte. 3 so there is a 5->3 lane merge there, then almost immediately after that Rte. 129 empties two lanes, leading to another 5->3 lane merge.
By the time you get past Treble Cove Rd, though, it will usually start moving a little, as the merges are more reasonable from that point on.
Rte. 62 to Rte. 128
The primary source of backup here is the off-ramp to Rte. 128. An additional lane breaks away from Rte. 3 on the on-ramp to Rte. 128 South, so two lanes go north, and two go south. However, the ramp to 128 North collapses to one lane soon after the lanes split. In addition, southbound traffic merges fairly quickly with often heavy traffic traveling south on Rte. 128 already.
It's rare to see problems here in the evenings.