l just install mikrotik routerOs my pc with a card and l want to use 3g usb modem and with Linksys router to setup hotspot
You can follow this step.
- Make sure your 3G modem supported by mikrotik. For supported hardware, read this article : http://wiki.mikrotik.com/wiki/Supported_Hardware
- 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_Globetrotter_HSDPA_USB_Modem
- When the router is already connected to the internet . Setup your Hotspot : http://wiki.mikrotik.com/wiki/Hotspot_server_setup

If you want to use external Access Point, you have to set your external Access point as bridge.