detecting connectivity betweend pc and modem using mscomm and only 3 wire rs232

What kind of interface/device are you using that requires/uses only 3 wires?  I am assuming you are using software flow control other wise you can't even communicate reliably.

As, reswobslc stated, with only TX and RX you really can't tell because you don't have access to the signals/leads that tell you the status.

Actually, 3 wire RS232 is very common.  Just not with modems.

A lot of computer-to-computer links are 3 wire RS232.  Flow control just doesn't make sense much of the time - computers with gigahertz and gigabytes to spare aren't in a position where they are going to boil over receiving 10 KBytes/sec if it can't signal the other guy to slow down.

Modems don't even strictly need flow control either for many applications.  They only started needing it the day they implemented data compression and error correction and offered the ability to use a different bps rate between the modem and computer than on the phone line, when the actual throughput wasn't known in advance.  Even then, many protocols that still use modems simply don't send enough data to ever overflow a buffer.  For example, when a modem is used to validate a credit card transaction, the packets containing the credit card number and dollar amount, as well as the packet containing the approval/decline, are so small the buffer would never overflow, flow control or not.  And 3-wire RS232 controls lots of gadgets inside devices, like LCD displays, keypads, servo controllers, barcode readers, magstripe readers, you name it, and these don't use flow control because they are constrained to processing only tens or hundreds of bytes at a time by their nature and design.

you were right i can't do it reliably