343=11在数字中放一个符号，使等式成立。
(1)(1+2)÷3,
=3÷3,
=1;
(2)1×2+3-4,
=2+3-4,
=1;
(3)[(1+2)×3-4]÷5,
=[9-4]÷5,
=1;
所以，答案是:(+)，；×,+,-;[(+),×,-],÷.
=3÷3,
=1;
(2)1×2+3-4,
=2+3-4,
=1;
(3)[(1+2)×3-4]÷5,
=[9-4]÷5,
=1;
所以，答案是:(+)，；×,+,-;[(+),×,-],÷.