The Samaritans were not really Jews: they were more like semi–Jews. Here's how it happened.
At one time, the land of Israel (and therefore, the children of Israel) was divided into two kingdoms: Israel in the north, and Judah in the south. Hoshea, king of Israel, did evil in the sight of the LORD. So God sent the Assyrians to destroy Israel (cf. 2Kings 17, Isaiah 10:5-15). The Assyrians took the people of Israel, and relocated them in Mesopotamia, and took people from Mesopotamia, and relocated them in the land of the northern kingdom of Israel. These two groups in the land of Israel intermingled (and inter–bred), and mingled their various gods with the LORD.
2 KINGS 17:33 They feared the LORD, and served their own gods, after the manner of the nations whom they carried away from thence.
2 KINGS 17:41 So these nations feared the LORD, and served their graven images, both their children, and their children's children: as did their fathers, so do they unto this day.
(You can't do both). The people formed from this were called "Samaritans," and the northern part of the land of Israel was subsequently called "Samaria." After this time, only the Kingdom of Judah remained (2Kings 17:18...that's why they're called "Jews" today). We know from John 4:9 that the Jews had very little to do with the Samaritans (since the Samaritans were "polluted").
By the time of Christ, the Samaritans did in fact inhabit part of the Land of Israel, the land that God had promised to the patriarch Abraham. Additionally, although the Samaritans were descendants of mesopotamians, they were also descendants of Jacob. So when the Gospel of Jesus Christ was to be preached among all nations, beginning at Jerusalem, it makes perfect sense that the Samaritans would be the second group to hear, right after the Jews (compare the circumcised Samaritans, who "feared the LORD and served their own gods," with the un–circumcised Greeks and Romans.
The first seven chapters of the book of Acts takes place at Jerusalem. In the eighth chapter of Acts, the Gospel is taught to the Samaritans!
ACTS 8:5 Then Philip went down to the city of Samaria, and preached Christ unto them.
ACTS 8:12 But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women.
ACTS 8:13 Then Simon himself believed also: and when he was baptized, he continued with Philip, and wondered, beholding the miracles and signs which were done.
ACTS 8:14 Now when the apostles which were at Jerusalem heard that Samaria had received the word of God, they sent unto them Peter and John:
ACTS 8:15 Who, when they were come down, prayed for them, that they might receive the Holy Ghost:
ACTS 8:16 (For as yet he was fallen upon none of them: only they were baptized in the name of the Lord Jesus.)
ACTS 8:17 Then laid they their hands on them, and they received the Holy Ghost.
9.3.1 Water baptism is part of the gospel of Jesus Christ. Today, many churches say "We preach Jesus," but they do not teach that baptism is for the forgiveness of sins (Acts 2:38), and is required for salvation (Mark 16:16). According to Acts 8:5, Philip "preached Christ" unto the Samaritans.
ACTS 8:5 Then Philip went down to the city of Samaria, and preached Christ unto them.
Acts 8:12 mentions some of the things that they finally believed, namely, "the things concerning the kingdom of God, and the name of Jesus Christ."
ACTS 8:12 But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women.
Since they believed these things, Philip must have preached these things (why else would they even think of them). Likewise, when they believed, they acted according to that belief: they were baptized. Therefore, water baptism was also among the things that Philip preached. When Philip "preached Christ unto them" (Acts 8:5), these were among the things that he taught.
9.3.2 The Samaritans were baptized in the name of Jesus Christ. Philip preached the name of Jesus Christ. Acts 8:12 specifically states that the Samaritans believed the name of Jesus Christ. The phrase "in the name of the Father, and of the Son, and of the Holy Ghost" is not present (nor is it present anywhere in the book of Acts, for that matter. See section 6). "Jesus Christ" is the only name mentioned. With this in mind, along with the precedent stated at Acts 2:38, the only reasonable, and honest conclusion that can be made is that the Samaritans were baptized in the name of "Jesus Christ."