Required ratio = 3 :10
In 1st alloy $1 : 2 =3$
In 2nd alloy. $2 : 3 =5$
In 3rd alloy. $5 : 8 = 13$
LCM of $3, 5 ,13$ is $195$
In 1st alloy$ 65:130$
Since $3\times65=195 $
So the wt of 1st alloy is$ 65$
therefore $1 \times 65 : 2 \times 65 = 65 : 130$
Similarly, in 2nd alloy, $78:117$
Since$ 5 \times 39=195$
So the wt of 2nd alloy is$ 39.$
Therefore$ 2 \times 39 : 3 \times 39= 78 : 117$
In 3rd alloy, $75:120$
By alligation method,$ 78-75=3 75-65=10$
Therefore the ratio is $3:10$