Hi aryanc.  I'll try my best to help ya. ok

I recently purchased the Line 6 spider 3 30watt , firstly let me say that this is a fine amplifier in terms of sound and tone , looks ( really stunning ) and ease  of controls , For a basic amp this sure is loaded .... I absolutely love it ,  anyway on to my queries...

Glad you like your amp.  I don't have this same amp so my answers my be slightly off.  ok

While testing the unit at the store itself , I had to swtich on and off amplfier and the sound came on the second time unit being powering up  , on switching on the first time there was no sound) only the Clean channel light comes on) , I'm having to do this each time I start it ,  in order to  get any sound  (i.e switch on and off and then on again) .I really dont mind switching it on twice since I love this amp so much , but it doesnt really make practical sense having to switch on the amplifier twice .

I can not answer this question.  My only thought is "are you letting it warm up?"  Maybe it takes a moment to warm up.  I don't know this one. Sorry.

Now when the system does come on ( with all the knobs turned down )  the FX are activated and the tap light starts flashing  , turning the Echo FX knob up and down to zero  resets the effects to none  ...so I reset all the settings to the default factory settings as indicated in the manual ...its still the same.

When the amp first comes on it "remembers the last setup" and will return itself to those settings.  The knobs on the front of the amp means nothing because they are not actually hooked to anything (like we think).  The knobs are only a reference to the internal brain on the amp, probably called the DSP.  The DSP simply senses the position of the knobs when you turn one then sets the parameters to match how much you turned the knob.  I hope this makes sense....  If these knobs were actually hooked to a piece of hardware then the DSP could not change the values it needs to change when you select a different preset or amp model.  With them being only simulated knobs lets the DSP control all the functions.  I think what you are seeing is pretty normal for Line6 products.

If you create your own tone and save that tone as a new preset then the amp will turn on with that preset automatically selected, instead of whatever preset its coming on with now.

Also when I switch from the Clean channel  to the other channels  Crunch , Metal, Insane ...the apparent channel sound volume increases. each time I change it gets progressively louder from Clean to Crunch to Metal and Insane , turning the channel volume down and back up solves this issue  ...is this normal ??

With Line6 products I can say "Yes this is pretty normal" by what I've seen and heard  lol.   Some will say its because those models are models of amps that are running higher gain settings, and maybe some other stuff also, but I've said in the past it's because none of these amps were ever actually tested before they were built.  I don't know the correct answer as to why all their products is like this.

Again, you need to create and save some presets of your own and use them instead of the factory presets and models.  That way you can set the channel volume leves to your needs and not be blasting yourself everytime you hit a button.

And again, the channel volume is not actually hooked to anything, its only a reference for the DSP.  It must be like this for the DSP to provide you with all that volume you get when you change those model and presets.   If you amp has a Master Volume then you can do like me and first turn it completely down then change the factory presets/models then come back up on the master volume.  That way you won't get blasted. lol

(the tap delay light comes on too when selecting to metal and Insane)  ..and when I come back to the clean channel the sound is really low and the effects are activated again and the tap light starts flashing  ,  turning the channel volume and Echo FX control solves this.

The tap tempo light will flash all the time.  I don't think it can be turned off.  That is normal.

You just need to remember this amp is controlled by a brain, not the knobs on the front of the amp.  With a brain contolling it you must learn how to use it to your advantage and also learn its little quarks.  I suggest you come here and read when you can and also read and experiment with your manual and your amp.  It all gets easier once you know exactly whats going on in there.

Hope this helps.  My fingers are tired now. lol

Good luck man.  And keep playing!!!!!!!!