Bombardier Challenger 300/350/3500
The Challenger 300 series is essentially a scaled-down version of the Challenger 600 series, so there are similarities. It remains an aircraft with low mounted, swept wings, a T-tail and engines attached to the rear fuselage on pylons. The nacelles have a single exhaust. The cockpit windows, which become narrower towards the end/side, are a typical recognistion point. To differentiate between the subtypes you have to look at the winglets.
Bombardier BD-100-1A10 Challenger 300
The first version of the BD-100 has winglets attached almost perpendicular to the wings. This is the only external difference compared to the Challenger 350.
Bombardier BD-100-1A10 Challenger 350 & 3500
The second and third iterative of the super mid-size Challenger have winglets attached to the wings at an angle of about 45 degrees. These winglets are a bit larger than on the Challenger 300. This is the only external difference compared to the Challenger 300.
The Challenger 3500 is externally the same as the 350, but has an improved cabin, reduced cabin altitude and an autothrottle system.
Confusion possible with
The Challenger 300/350 looks most like its larger sibling, the Challenger 600 series. Look at the cockpit windows to keep them apart. They keep the same height at the 600 series, and become narrower towards the end at the 300/350. Also on all large cabin Challengers - except the 600 and 600S - the engines have a separate fan exhaust, while the Challenger 300/350 has a single exhaust.
Raytheon Hawker 4000
Being similar in size you might mix-up the Challenger 300/350 and the Raytheon Hawker 4000. These can be distinguished by the fairings for the thrust reversers: external on the Hawker 4000, not visible on the Challenger.
Cessna 700 Citation Longitude
The Citation Longitude is also similar in size as the Challenger 300/350. To keep them apart look at the differences in cockpit windows, apart from more subtle differences.
Embraer 550 Legacy 500
The final aircraft of similar in size is the Embraer Legacy 500. The main difference is in the top of the vertical stabiliser. At the Embraer it appears to be tilted forward.