关于文件系统挂载问题

在公司有台服务器，在运行命令
oas35:/opt # fdisk -l

Disk /dev/sda: 36.4 GB, 36401479680 bytes
128 heads, 32 sectors/track, 17357 cylinders
Units = cylinders of 4096 * 512 = 2097152 bytes

Device Boot Start End Blocks Id System
/dev/sda1 * 1 1 353 41 PPC PReP Boot
Partition 1 does not end on cylinder boundary.
/dev/sda2 52 2100 4196352 82 Linux swap
/dev/sda3 2101 17357 31246336 83 Linux

oas35:/opt # df -k
Filesystem 1K-blocks Used Available Use% Mounted on
通过df -k，发现没有挂载的文件系统，于是，执行
oas35:/opt # mount /dev/sda3 /----------------报错
mount: /dev/sda3 already mounted or / busy

请问这是什么问题，万分感谢！