Let's focus on the similarities first. All three churches -- Catholic, A/G and Baptist -- believe this:
1. Jesus is God
2. God is in three Persons: Father (Creator), Son (Jesus) and Holy Spirit.
3. There is only one way to heaven, and that's by the blood of Jesus.
4. It's right to follow the 10 commandments.
5. It's right to read and follow the Bible.
6. It's right to do the work of an evangelist.
...and so on. There's more similarities than differences, actually.
It's hard to define all the exact differences because there are different kinds of Baptist churches and in the A/G, there are some disputes (I used to work for one of the 12 executive presbyters in the A/G, so I had first-hand knowledge of this).
Some things that are shared by Baptists and A/G but not Catholics:
1. A person has to choose to be saved.
2. A person can only be baptized after they have met certain criteria (Praying a certain prayer, giving a testimony, etc.).
3. Baptism is the only Sacrament.
4. Communion is optional and it's just a symbol and not that big of a deal.
5. There will be a rapture of believers, leaving devastation as people are swept up to heaven from their jobs, cars, etc.
6. Sola Scriptura (even though it's not in the Bible and they don't actually follow it but have all sorts of things in addition to Scripture)
7. And so on (there is more).
Some things that are shared by the A/G and Catholic Churches, but not Baptist Churches:
1. A person can lose his/her salvation
2. Christians are free to associate with non-Christians (some Baptist churches say their people can't even associate with non-Baptists!)
3. And so on.
Some things that are unique to the A/G
1. A person isn't really saved unless they have the gifts of the Spirit, specifically tongues.
2. Once a person is "filled with the spirit" they can control what the Holy Spirit does and what gifts He bestows onto what people
Some things that are unique to Baptists
1. Once saved, always saved.
Some things that are unique to Catholicism:
1. Apostolic Succession (an unbroken chain of leadership since Peter was commissioned by Christ)
2. Seven Sacraments (baptism, reconciliation, communion, confirmation, holy matrimony, holy orders, annointing of the sick)
3. Communion of Saints (that is, Catholics believe that the saints in heaven are still part of the Body of Christ and therefore are still praying for God's will to be done on earth as it is in heaven)
4. The perpetual virginity of Mary
5. The Holy Eucharist transubstantiated from bread and wine into the true Body and Blood of Christ
6. And so on.
We should just all agree -- I think so, too. That's why I got suspicious of denominationalism and embarked on a 6-year study to find the true Christian Church. Which is why I'm Catholic now.