The survey in Iowa shows the Democratic nominee holding a slim 41 percent to 37 percent lead over her Republican rival. In July, Clinton led Trump in Iowa by 3 percentage points.
In Ohio, Clinton leads by 43 percent to 38 percent — a 5-point shift since July ,when the two candidates were tied.
In Pennsylvania — like Ohio a key Rust Belt target for Trump — he trails by a double-digit margin. Clinton draws 48 percent to 37 percent for Trump. Prior to the conventions, Clinton led by 9 points over Trump, 45 percent to 36 percent.
More worrisome in the Keystone State for Republicans trying to hold their Senate majority, GOP incumbent Pat Toomey also trails the relatively inexperienced Democrat challenging him, Katie McGinty, by 48 percent to 44 percent.
The Iowa telephone poll of 899 voters carries a margin for error of 3.3 percentage points, the same margin for error as in the Ohio survey. The Pennsylvania poll of 834 voters carries an error margin of 3.4 points. All three were conducted Aug. 3-7.