In Host PC that have USB Type C Ports is it correct to assume that they will be implemented with PD controllers
It is up to manufacturer whether they support PD or not. Having said that, I suspect all new devices with Type C will support it.
Use the CC lines to decide whether to provide VBUS (and in the worse case may even not provide VBUS power)?
Since Type C is backwards compatible it has to provide at least 500mA for old devices without the use of CC lines. I don't know whether you can use CC to negotiate cutting yourself from any power. It makes little sense so I suspect designers did not even plan for this possibility.
The above statement was based upon USB PD 3.0 specification. Following the comments by @AliChen I checked Type-C 1.3 specification. Apparently, Type-C ports implement "USB Type-C-specific implementation of the USB Power Delivery Specification". And later: "USB Power Delivery optimized for the USB Type-C connector".
Specifically, Type-C port does not supply 5V until it detects attached sink device and determines its type. So, yes, in worse case it may not provide VBUS (or VCONN) power.
Note that this behavior is allowed by PD spec, which states that downstream port can source either 5V or 0V in its initial state. I suspect that 0V option was added to PD 3.0 in a hurry to accommodate Type-C spec. First, because "source 0V" sounds idiotic, and second, because in some parts of the spec it still mentions only 5V and requires DFP to tolerate 5V supplied from the other side.
Once proper validation with the device connected then will be able to provide +5V upto 3A on the VBUS lines?
If you have port with PD 2.0 and up then yes. For PD 1.0 it's 2A. In either case "You're gonna need a bigger cable". But if you need more power you can try negotiate for 20V 5A, although it is my understanding the host may limit output power to whatever their circuitry can provide.