Android uses the Linux kernel instead of some "Android kernel", so why is ext3 not supported on Android 2.3?
The filesystem support is device-specific, and in fact many devices using Android 2.3 support ext3 in the kernel (or ext4, which can also mount ext3 and ext2 filesystems).
Usually the difference in filesystem support is due to different hardware. Older devices often used raw NAND flash chips and MTD drivers in Linux, which did not support conventional filesystems such as ext4; only special filesystems such as YAFFS or JFFS2 could be used on such devices. In this case having ext3 or ext4 support in the kernel was not necessary, and it could be omitted to save some memory; to get ext3/4 support on such device you will need to flash a custom kernel (or, in some cases, an external module could be compiled instead of replacing the whole kernel).
More recent devices usually use eMMC storage, which provides a block device instead of raw flash; such devices mostly use ext4 for their filesystems, and therefore should have support for mounting ext3 filesystems (although actually using it to mount a custom filesystem, e.g., on a separate SD card partition, will require root access).