Well, for starters the Valkyre coup as we know was badly planned, and it should be noted the 20 July version was probably as best prepared as it would ever be in terms of seizing power. July 15th is often cited as a "missed chance" (particularly since Rommel hadn't been wounded), but it should be noted that given the lack of coordination between Stauffenberg and the Berlin plotters in that day, Goering and Himmler wouldn't have been killed either. To make matters worse, attempts to put Berlin troops in alert to prepare Valkyrie failed and proved to be ridiculously flawed, giving Olbricht and company the chance to refine plans for July 20th (which still were badly implemented). Fromm was also with Stauffenberg that day, and since Fromm is not jumping on a plane with the Colonel to raise suspicions about himself the conspiration can't even get Fromm's authority to back them up if Hitler actually dies, meaning the Reserve Army and many of the military districts probably refuse to follow orders.
Like it has been pointed out, 1943 and even early 1944 provide good chances of killing Hitler, Himmler, and Goering at once. The irony, I believe, is that the Valkyrie plotters would not be ready to seize power, having less support, less preparation and less means than IOTL as they haven't infiltrated the Reserve Army as throroughly (and without the Reserve Army the coup is nothing), and it doesn't look like most of the field commanders of the time would back the coup unless it had already been succesful. But if Himmler, Goering and Hitler are indeed dead, then the chances of the plotters are dramatically increased, and if they get enough luck to get the Valkyrie orders to the acknowledged by most commanders over the first hours along with suitable backing from a high-ranking figure (they absolutely need Fromm at the very least), then I suppose there is a chance. The SS has been decapitated and while it can do damage, it doesn't really have a figure of their own to rally behind. With Goering gone you lose the figure of the "designated successor" that could rally enough support from enough key figures.
You're then left with Bormann, who is hated and unpopular, Goebbels, who doesn't a chance himself unless he manipulates as suitable puppet or joins forces with others, Speer, who is at his highest point of influence as a possible successor to Hitler in late 1943 but in the middle of a nervous breakdown and a health crisis in early 1944, and the Wehrmacht commanders, with Rommel having the sort of popularity needed and yet not having been informed of the plot during this time span.