[SilentSoul]

Ok i have an idea for this , let's clear the chat for the player and send him that message , try that should work.

pawn Код:

public OnPlayerConnect(playerid)
{
    for(new i = 0; i < 50; i++) SendClientMessage(playerid,-1," ");
    //SendClientMessageToAll(....) add your message here.
    return 1;
}

or another idea create a timer within 3 seconds and after the 3 secs passes send the clientmessage.

pawn Код:

public OnPlayerConnect(playerid)
{
    SetTimer("Message",3000,false);
    return 1;
}
forward Message();
public Message()
{
    //sendclientmessagetoall....
}