3 PARALLEL LM350's NOT WORK Properly.
Additionally Maybe not even one.
You Don't say What the Output Voltage of your Transfomer is?
First, At 9 Amps, You better have some Very Large Filter Caps to reduce ripple.
Second, These Devices are Rated at a Specific Amperage, Like 3 Amps for the LM350, BUT they are Also Rated for a Maximum Wattage. If the Wattage EXCEEDS its Limit, It will NOT HANDLE THE RATED AMPERAGE.
Third, You Better have some VERY GOOD HEATSINKS, no matter which way you do this. Or they WILL BURN UP.
Don't know where you got a price of $6.00 each, That is Awfully High Priced. I buy them for Under $3.00 each Canadian. (About $2.25 US)
Here is the Proper way to Increase Current of either the 317 or 350.[/quote]