Answer
$A=8,12,19,34 ; B=3,5,15,21$
$3<8 \Rightarrow A=8,12,19,34 ; \quad B=5,15,21 \quad C=3$
$5<8 \Rightarrow A=8,12,19,34 ; \quad B=15,21 \quad C=3,5$
$8<15 \Rightarrow A=12,19,34 ; \quad B=15,21 \quad C=3,5,8$
$12<15 \Rightarrow A=19,34 ; \quad B=15,21 \quad C=3,5,8,12$
$15<19 \Rightarrow A=19,34 ; \quad B=21 \quad C=3,5,8,12,15$
$19<21 \Rightarrow A=34 ; \quad B=21 \quad C=3,5,8,12,15,19$
$21<34 \Rightarrow A=34 ; \quad B=\quad C=3,5,8,12,15,19,21$
only 34 left $\Rightarrow \quad C=3,5,8,12,15,19,21$
Work Step by Step
$A=8,12,19,34 ; B=3,5,15,21$
$3<8 \Rightarrow A=8,12,19,34 ; \quad B=5,15,21 \quad C=3$
$5<8 \Rightarrow A=8,12,19,34 ; \quad B=15,21 \quad C=3,5$
$8<15 \Rightarrow A=12,19,34 ; \quad B=15,21 \quad C=3,5,8$
$12<15 \Rightarrow A=19,34 ; \quad B=15,21 \quad C=3,5,8,12$
$15<19 \Rightarrow A=19,34 ; \quad B=21 \quad C=3,5,8,12,15$
$19<21 \Rightarrow A=34 ; \quad B=21 \quad C=3,5,8,12,15,19$
$21<34 \Rightarrow A=34 ; \quad B=\quad C=3,5,8,12,15,19,21$
only 34 left $\Rightarrow \quad C=3,5,8,12,15,19,21$