I do not know of any Scripture that says you must believe in the trinity, there is Scripture that proves the trinity,which the Holy Spirit eventually teaches a saved person that the three are in fact, one. "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son." - 1 John 5:7-10.
And I know that your question is not about that, but personally, when I give the plan of salvation I am speaking about Jesus alone, the only perfect Son of God. No mention of Him being God and the Holy Spirit ever comes up, but no one questions it either when I witness to them. If people believe that Jesus is the perfect Son of God they will eventually understand He is God as they grow. So, technically, I believe that a person can be saved by believing in Christ alone, if they believe that He is the only perfect Son of God, whom died on the cross, was buried, and arose from the dead and they are trusting in Him alone to save them then I believe they can be saved. If they do not believe in the Gospel, then no, they will not be saved.