  3$\alpha$-cluster structures and monopole transitions of $0^+$ states in
$^{12}$C and $^{14}$C were investigated with $3\alpha$- and
$^{10}$Be+$\alpha$-cluster models. A gas-like $3\alpha$ state and a
bending-chain $3\alpha$ state were obtained in the $0^+_2$ and $0^+_3$ states
of $^{12}$C, respectively. In $^{14}$C, a linear-chain 3$\alpha$ structure is
found in the $0^+_4$ state near the $^{10}$Be+$\alpha$ threshold, but a cluster
gas-like state does not appear because valence neutrons attract $\alpha$
clusters and suppress spatial development of 3$\alpha$ clustering. It was found
that the linear-chain state in $^{14}$C is stabilized against the bending and
$\alpha$ escaping modes by valence neutrons. The monopole transition strengths
in $^{12}$C are enhanced by $3\alpha$-cluster developing, whereas, those in
$^{14}$C are not enhanced so much because of the tight binding of $\alpha$
clusters by valence neutrons.
