The Gemara (Berachot 4b) relates that whoever recites Tehila LeDavid (Mizmor 145) thrice daily is guaranteed a place in the World to Come. Accordingly, the Sages incorporated this psalm three times during the day; during Pesuke DeZimra, before Uva Letzion and prior to the Minha Amida. Some have the custom to recite the verse “Yehi Hasdecha Hashem Alenu Ka’asher Yihalnu Lach” before the psalm, however, according to the Livorno rite (upon which many Moroccan customs are based), this verse only is said before Tehila LeDavid of Shabbat Musaf.
The Avudraham explains that the mere fact of reciting the Mizmor three times is not what guarantees the World to Come, but rather saying it intently. Therefore, by saying it three times daily, it assures that it will be said intently at least once. Furthermore, the Bet Yosef explains that Tehila LeDavid itself does not necessarily guarantee Olam Haba, but rather that if one’s merits and one’s sins are equal, reciting the Mizmor intently can tip the proverbial scale in one’s favor. The Shibole Haleket says that the verses of “Hanun VeRahum” and “Karov Hashem” allude to the word “Hok”, which has an implication of permanence and also has the same numerical value as “Gehinom”. This alludes to the fact that one who recites the Tehila LeDavid three times daily will
not inherit the judgement of Gehinom.
Summary: It is praiseworthy to recite Tehila LeDavid three times daily.