Seems to me that a 3 year lectionary can cover far more of the Bible than a 1 year. There are church attendees who do not read the Bible on their own, and church readings are all they get.
In the days where Matins and Evensong where the mainstream liturgical diet of the lay folk, with Communion on High Days and Holy Days, the congregation was engaged with both Old Testament and New Testament readings, and a good dose of the psalter on a regular basis, and the the Communion accounted for an Epistle and Gospel Reading seemed sufficient for a New Testament Sacrament. Many folk these days hardly ever encounter Matins or Evensong, so the three year lectionary makes sense in that it does introduce the breadth of scripture to the person in the pew. The question of course then becomes which three year lectionary and how do you fashion it. In Australia for some time we widely use the Revised Common Lectionary, which was essentially a de-romanised Roman Lectionary. Since then we have engage the Australian Three Year Lectionary which was modified to provide for sequential OT readings. The other side of that problem is that often the OT reading does not relate well to the other readings without some for fearsome eisegesis on the part of the preacher, who seem to feel that they need to make the whole thing tie up neatly.
Very little. As Boltoph said, the point of the 1-year lectionary was not to cover lots of Scripture, but (as I would put it) to cover the Gospel. The Daily Office Lectionary is where the Bible's full text (well almost) is read. But, as Boltoph also pointed out, people don't really do the Office anymore, and so there is a pastoral sense to using the modern 3-year lectionaries. (Yes there are several of them, but most of them are closely related.) I wrote up my own opinions on the matter a couple years ago here: https://leorningcniht.wordpress.com/2017/11/07/my-top-3-reasons-to-ditch-the-rcl/ and there's a document at the end which I'd sent to the Prayer Book committee before it was finalized.
I do not think it matters. Perhaps the experience of others is different to mine. I believe if we went to coffee after Sunday Mass (do you remember when that used to happen?) I think the majority would have forgotten what the days readings from the Bible had been. I think people get more out of the Bible by either reading it themselves or by going to Bible Study.
People learn by repetition also. Hearing the same scripture every 3 years should cause some of it to sink in! Better than nothing, anyway.
I don't think people usually have three-year memories. If the repetition is going to have an impact, it needs to be yearly, in my opinion.
My sister complains that every time she goes to Church they have the same readings. I pointed out to here that if she wanted to here more readings, perhaps she should go to Church on a day other than Christmas!
The 28 American has two sets of Propers for Easter. I bend the rubrics a little and use the second set sometimes. They are intended for a second celebration but we never schedule 2 services on Easter. The second set of readings gives me Mark 16. My rector is one who makes a huge issue about the authenticity of the ending of the book and generally ignores Mark most of the time.
Scriptures I haven't read or heard in 3 years (or more) come to mind in response to circumstances. But then, the Holy Spirit is probably the one bringing them to mind.
One year. It worked fine for over a thousand years and fit within a cycle that most humans can think in terms of.