| Quote #1
I was of three minds,
The expression "to be of two minds" is a commonly used idiom that means "to be undecided between two opinions or desires."
| Quote #2
I do not know which to prefer,
We don't think the speaker means to suggest he's actually torn between these two things. The problem doesn't keep him up at night. What he means is something along the lines of, "The whistling of the blackbird is really beautiful, even when it's not whistling."
| Quote #3
O thin men of Haddam,
The men of Haddam choose to ignore the blackbird in favor of a mythical "golden" bird that they'll never be able to find, unless they know something that the rest of the world doesn't. The poem implies that they ignore the lovely feet of the women around town, because if they were looking at the feet, surely they would have noticed the blackbird.