My question is in three parts;
a) did Lee ever have three such bad days in a row during the ACW............Not sure he actually had three bad days in a row. Day one was a huge success, First Corps, the cream of the Union Army, specifically The Iron Brigade, took a pounding. The Iron Brigade was never the same after Gettysburg, they left it all on the field, specifically their stand at the Seminary, which allowed the rest of the Union army to reform and regroup. Eleventh Corps was pushed back and through the town as well. So in short, the Union Army was driven from the field, I'd call that a success.
Day two was a near run thing; on the Union left flank Devils Den was taken, Little Round Top was nearly taken, the wheatfield was taken, the peach orchard was taken, shattering the Union 3rd Corps in the process, the Confederates drove all the way to the Trostle Farm where a stand by the 9th Massachusetts held them off, and Wrights Georgians got all the way to the crest of Cemetery Ridge, driven off by the 20th MA and 13th VT. Wilcox's Alabamians almost got there too, but were driven off by the famous charge of the 1st MN.
On the Union right flank, parts of Culps Hill were taken and the Confederates got all the way to the crest of Cemetery Hill but were driven off in vicious hand to hand combat in almost complete darkness. So overall, day two was not a total disaster, although the Union line did hold at the end of the day.
Day three; again, it's been discussed lately that Lee's actual target was further to the north and if it has hit there and was coordinated with Ewell's attack on Culps Hill, it might have worked.
b) what were the main reasons for his defeat at Gettysburg do you think.................The main reason for his defeat was he had grown to believe his troops could do anything he asked them to do, Picketts Charge proved that not to be the case.
c) how big a factor was Ewell's refusal to attack Cemetery Hill on day 1 in the defeat at Gettysburg?................It's really not his refusal to do so, he was not ordered directly to attack and did not, it was more his belief that his men were spent after fighting for most of the day. Was it more Lee's fault for not directly ordering him to attack, that is the real question........
Hope this helps.