Q – I am not sure if I am missing something, does it say anywhere, that the apostles were baptized in water? (I know they were baptized by the Holy Spirit, but were they baptized in water?)

A – We know from Romans 6 that the apostle Paul was baptized because he uses the pronouns we and us in describing the experience of baptism. And his conversions story, including baptism, is recorded in Acts 9 and in Acts 22. I realize that your question was primarily about the other apostles, but we can see the same thing in 1 Peter 3:20-21 about Peter:

1 Peter 3:20-21 -- 20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. 21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:

This verse is obviously about water baptism, and Peter says that baptism saves us. So Peter was obviously baptized in water.

Q – Hebrews 6:4-6 - What are these verses saying in regards to falling away?

A – Heb 6:4-6 – 4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

This passage must be understood in light of the rest of scripture, most
notably 1 John 1:9. This verse says that any sin we confess will be forgiven. Hebrews 6 seems to indicate that some sin cannot be forgiven.

The only way I can see to reconcile this is that the sin in Hebrews 6 is one for which the sinner does not repent and confess. In fact, Hebrews 6 says it is impossible to renew the people under consideration to repentance.

I believe that Hebrews 6 is referring to people who become callous to the point that they cannot be touched and brought to repentance. This passage stands as a warning to those who wander away from the lord. You can go so far down that road that your heart cannot be brought back. It is not that the Lord will not forgive the penitent sinner. It is that the callous sinner will not repent.

Q – What is meant by "the third heaven" in 2 Cor 12:1-4?

A – The Bible uses the word heaven to refer to 3 different things:

1.   It refers to the atmosphere around our earth, the sky in which the birds fly. This is the first heaven.
1 Kings 18:45 – And it came to pass in the mean while, that the heaven was black with clouds and wind, and there was a great rain. And Ahab rode, and went to Jezreel.

2.  It refers to outer space where the sun, moon, and stars are. This is the second heaven.
Genesis 15:5 – And he brought him forth abroad, and said, Look now
toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.

3.   It also refers to the spiritual realm, or the place where God is. This is the third heaven.
1 Kings 8:30 – And hearken thou to the supplication of thy servant, and of thy people Israel, when they shall pray toward this place: and hear thou in heaven thy dwelling place: and when thou hearest, forgive.
Of course, the description of where God is is figurative language, for God is everywhere and is not a located being (Psalm 139:7-10)These three things are the three heavens.