Step 1: Old ratio.
X : Y : Z = 3 : 4 : 3.
Total = 10.
So X = 3/10, Y = 4/10, Z = 3/10.
Step 2: After retirement.
Y retires → his 4/10 share is to be divided.
X and Z share equally = 2/10 each.
Step 3: New shares.
X = 3/10 + 2/10 = 5/10.
Z = 3/10 + 2/10 = 5/10.
Step 4: Ratio.
X : Z = 5/10 : 5/10 = 1 : 1.
Final Answer:
\[
\boxed{\text{1 : 1}}
\]