Develop a musical piece using the given theme and incorporating the letter-based structure provided.
X:1
L:1/8
Q:1/8=120
M:6/8
K:G
['d GAB cBc edc BAG cBA d2', 'efg efg', 'c3 efg B3 cBA d2 c B>AB']  
X:1
L:1/8
Q:1/8=120
M:6/8
K:G
"^A" d | GAB cBc | edc BAG | cBA d2 c | B>AB G2 ||"^B" d | efg efg | efg afd | d>ed def | gfe dBG | 
 GAB cBc | edc BAG | cBA d2 c | B>AB G2 ||"^C" d | g3 c3 | efg B3 | cBA d2 c | B>AB G2 ||  