Smokey,
While Paul pretty much answered your question, the subject of starting out on only 3 rows is addressed elsewhere in the forum. Pretty much all the "Method" books take this approach. I u-tubed numerous master accordionists and discovered the majority of them pretty much stuck to the three outer rows with only occasional forays to rows 4 & 5. To my way of thinking, this was a waste of buttons. CBA playing is largely a game of patterns. As mentioned above, moving the pattern (tune) to a different root yields a different Key while still fingering with the same pattern.
The problem that emerges is the student learns the pattern for the tune, and changing said pattern in order to incorporate alternate buttons requires a re-learn of the pattern. What's the point? becomes the question. If you can play the tune on 3 rows, why complicate matters by changing what is already ingrained in your muscle memory? This accounts for why many master players only utilize 3 rows, but begs the question "Why not play a 3 row CBA instead of lugging around an extra 2 rows of weight and complication?"
Having 5 rows available on my accordion, I elected to "Center" on the middle 3 rows, as opposed to "centering" on the outer 3. This provides two "neighboring" rows, one above and one below. Search "centering" for the full discussion. This approach has worked well for me, still provides transposition and I use all 5 rows as required. Others may disagree with this approach as it differs from all the method books I've seen.
Chords: Anyanka gave the solution for chord patterns. Cool part of CBA is you learn the pattern for a major Chord and all the major chords will have the same pattern. Learn the pattern for a minor chord and you've learned the pattern for all the minors. This is known as "transposition". If you are incorporating all 5 rows in your playing, there will be 4 options (patterns) for every chord. Two will be relatively easy to form, and two that will be more difficult (and less useful). Learn both of the easy forms and you will find them to be useful options for fingering dilemmas.
Once you get the basics down, start fooling around with inversions. They are simple to learn and will provide a quick and easy way to vary the sound of your playing. To "invert" a chord, you drop the lowest (or highest) tone, the C (or G) in a C chord, and add the same pitch you dropped, only an octive higher (or lower).
An example would be a C Major chord = C, F, G, with the C as the root. The first inversion up would become F, G & C (Down: G, C & F), the second inversion up would be G, C & F (Down: F, G & C), the third would be back to a root C, only one octive higher (or lower).
If you are playing a I, VI, V (see circle of 5ths) rotation (as in 12 bar blues), you can change the root chords to inversions, which will add variety to your sound while staying within the Key. Very useful and easy. The patterns all move over the button board in a similar fashion. You only need to learn the patterns and how they relate to each other.
Good luck and persevere!
Waldo