Speed 100Mbps , Download only 3Mbps ?

You can also check if your switches/servers/hosts are set to full-duplex.

also check if your computers are running 100mbit, most devices are set to auto/auto (speed/duplex) and can result in a 100-half duplex mode which also reduces your througput.

Just a quick reminder

If my pc using 100Mbps NIC half duplex, the max speed i can get only 50Mbps send & 50Mbps receive ... is it?

your tcp/ip uses some ip-windows to send data from one side to another... which results in having 75% usage of your 100Mbit.
Resulting in 9,3MB.

If i'm using 100Mbps NIC full duplex but my internal download speed only 3.8MBps , what make the speed slow down? I not really understand with "TCP/IP uses some ip-windows ...result 75%".. Can you explain more details?

Thanks

The half/full duplex issue is to do with how the connection handles sending and receiving. Let's assume your 100mb/s connection has a maximum possible throughput of 100mb/s. You won't get that in practice for a number of reasons, but let's ignore them for now. Any file transfer down that wire is going to contain elements of sending (bits of the file going to their destination) and receiving (acknowlegments for each section, maybe retansmit requests and other control stuff). With a half-duplex connection there is just the one communications channel which can either be transmitting or receiving but not both. So there is an overhead in making the turn-around and the transmitting throughput is limited by the enforced pauses while the circuit is being used to send the acknowlegments etc back to the sending end. With a full-duplex circuit a second parallel path is provided to handle the reply traffic so the sending path can continue without interruption. So you get better throughput, but since the 100Mb/s capacity is the maximum the circuit can carry you'll never get more than that. Some people say that a 100Mb/s full duplex circuit provides 200Mb/s but if you follow the above logic you'll see it's not really true