Hi,

l install mikrotik on pc without mikrotik card and l want to use 3g usb modem for internet connection and also use the as a hotspot with a linksys router. can anyone help.

You can follow this step.
1. Make sure your 3G modem supported by mikrotik. For supported hardware, read this article : http://wiki.mikrotik.com/wiki/Supported_Hardware
2. Dial 3G modem using PPP-Client, for example read this article : http://wiki.mikrotik.com/wiki/Huawei_EVDO or this article : http://wiki.mikrotik.com/wiki/Option_Gl ... _USB_Modem
3. When the router is already connected to the internet . Setup your Hotspot : http://wiki.mikrotik.com/wiki/Hotspot_server_setup
4. If you want to use external Access Point, you have to set your external Access point as bridge.