This should do it. It's much less hardcoded so you can change the channel count just by changing one constant.
PHP Code:
#define CHANNEL_COUNT 3
new g_isChannelUsed[33][CHANNEL_COUNT];
public MyCallback(id)
{
for(new i = 0; i < CHANNEL_COUNT; i++)
{
if(!g_isChannelUsed[id][i])
{
// Send message using channel 'i', or whatever
g_isChannelUsed[id][i] = true;
new taskData[2];
taskData[0] = id;
taskData[1] = i;
set_task(1.5, "Task_ResetChannel", 0, taskData, sizeof(taskData));
break;
}
}
}
public Task_ResetChannel(taskData[])
{
g_isChannelUsed[taskData[0]][taskData[1]] = false;
}