Do the laws of Arla apply to the fruits of a gentile owned tree? May a Jew eat the fruits of the first three years from a gentile owned tree?
The fruits of all trees contain the Biblical Arla prohibition, whether the tree is owned by a Jew or a gentile, and whether in Eretz Yisrael or in the Diaspora. [Even the Arla fruits of a gentile in the Diaspora is Biblically forbidden. Some Poskim however rule that the Arla fruits of a gentile in the Diaspora is merely Rabbinically forbidden.]
If one is unsure as to the status of the fruit?
The above law only applies regarding a fruit which one knows for certain to be Arla, however in any case that one does not know for certain, then in the Diaspora it is permitted to be eaten while in Eretz Yisrael it may be prohibited. This matter will be explained in greater depth in a future Halacha.
If a gentile who is selling fruit states that the fruit is from the first 3/4 years of planting, is he believed?
If a gentile who is selling fruits claims that the fruits are from Arlah or Neta Rivai [the first 3-4 years], he is not believed. This applies even if he mentions who he purchased the fruits from. [Nonetheless, the above only applies if there is reason to suspect that the gentile may lie about his fruits, to increase their quality in the eyes of the buyer. If, however, this suspicion is not applicable, and the gentile is making the statement out of mere casualness, then one should suspect for his words and avoid eating the fruit.]
 Michaber 294:8; Tur 294:8; Tosafus Kiddushin 36b; Rosh 62; Ran 15a; Mordechai 501; Semag Lavin 146; Rabbeinu Shimshon Arla 3/9; Rashba 1734; Rambam Maaser Sheyni 10:5; Radbaz 580; Birkeiy Yosef 294/6
Other opinions: Some Poskim rule that the Arla fruit of a gentile is not prohibited for a Jew. [Pirush Hamishnayos of Rambam Arla 1:3; See Beis Yosef 294 and Birkeiy Yosef ibid; The Rambam in Mishneh Torah ibid retracted from this opinion]
 Aruch Hashulchan 294:11
 Chasam Sofer 286, brought in Pischeiy Teshuvah 294/7
 Michaber 294:9
 Michaber 294:28; Braisa Yevamos 122a
 The reason: As he may be lying for the sake of making his merchandise sound of greater quality, as a younger tree produces better fruit than an older tree. [Taz 294:29; Yevamos ibid]
 Michaber ibid; Rabbeinu Yerucham
 Taz Y.D. 122:4; Shach 127:20; Aruch Hashulchan 295:45