...In Acts 2, none of those 3,000 Jews spoke in tongues, although they had heard the apostles speaking in at least 15 languages and dialects. That too was a sign to unbelieving Jews.
This is an assumption on your part. Many scriptures focus on only one thing, and are silent on others. This concept is true as seen throughout scripture.
Peter explained on the Day of Pentecost that the speaking in tongues that the crowd was hearing was the result of people being filled with the Holy Ghost. (Acts 2:4, 33) The experience was prophesied. (Acts 2:17-18) Joel's prophecy stated specifically that God's Spirit would be poured out on all flesh. The promise of being filled with the Holy Ghost was available to all, not just a few.
Also relevant, Acts 2:39 expresses that the promise is available to as many as the Lord God shall call. Peter goes on to tell the hearers to save themselves. The group believes Peter message and gladly does their part by obeying the command to be baptized in the name of the Lord Jesus. The promise that they could expect to be filled with the Holy Ghost could have taken place at that time. Or, later as seen in the experience of the Samaritan believers.
Furthermore, your premise that the speaking in tongues associated with the filling of the Holy Ghost was only done as evidence that God accepted the various groups of people is flawed. As seen in Acts 19, the same experience of speaking in tongues occurred when 12 individuals heard and believed Paul concerning aspects of the gospel message. They too spoke in tongues upon being filled with the Holy Ghost and also obeyed the command to be baptized in the name of the Lord Jesus.
Referenced Scriptures:
Acts 2:33 Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear.
Acts 2:38-42
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.
40 And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation.
41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.
42 And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers.
KJV
Acts 19:1-7
And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts came to Ephesus: and finding certain disciples,
2 He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.
3 And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism.
4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.
5 When they heard this, they were baptized in the name of the Lord Jesus.
6 And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
7 And all the men were about twelve.
KJV