The Maharal MiPrague (Nesivos Oilam, Nesiv Avodah, Perek 12) refused to recite the piyut "Machnisei Rachamim" mentioned in selichos, as we don’t ask requests or daven to malachim, "rather, we only daven to Hashem our G-d who listens to our tefillos".
However, the Sdei Chemed (Ma’areches Rosh Hashanah, siman 1) writes: Many great poskim advised to yes say it, as the intention of the piyut is that the malachim should advocate on our behalf and plead to the Ribbono Shel Olam on our behalf and we are not asking that the malachim themselves should answer our tefillos.
We find precedent to this idea in Sanhedrin (44b) where it says: "The heavenly angels should help us beseech for mercy." and Rashi explains this to mean: "The heavenly angels should help us beseech for mercy."
The Shibbolei HaLeket (siman 282) quotes the Medrash in Shir HaShirim on the pasuk "Knesses Yisroel says to the malachim standing at the gates of prayer: Take my tefillos and tears in front of the Ribbono Shel Olam, and be a good advocate for me."
Based on the above, we can understand that in the piyut "Machnisei Rachamim", we are not davening to the malachim, we are davening to Hashem, and we are simply asking the malachim to help us along and advocate for us.
Saying Viduy Three Times
The Levush (581:1) writes: "In certain communities the custom is to recite at the end of selichos viduy, 'Ashamnu Bagadnu...', three times."
The Rokeach (siman 207) explains that this corresponds to "Chatasi, Aviti, Pashati", which refers to three different types of sin, and the Kohen Gadol uses this nussach in his viduy on Yom Kippur.
However, the Vilna Gaon held that saying viduy once is enough (Maaseh Rav 202). This is also the opinion of the Arizal (Sha’ar Hakavanos pg. 51) and is also the minhag of Kamarna (cited in Minhagei Kamarna, siman 400).
