According to Wikipedia...
Penataceratops was about 8 m (27 ft) long, and has been estimated to have weighed around 5,500 kg (13,000 lb).
Individual Triceratops are estimated to have reached about 7.9 to 9.0 m (26.0–29.5 ft) in length, 2.9 to 3.0 m (9.5–9.8 ft) in height, and 6.1–12.0 tonnes (13,000-26,000 lb) in weight.
So they are pretty comparable in size.
I'm not sure what the three attacks represent, but I think it would be safe to just go with a gore like the triceratops.
Comparing game stats to triceratops:
Tri had 4 more HD, did more damage (1-8/1-12/1-12) and had a deadlier trample (2-12), and was 4 feet longer. Otherwise, they were identical.
Other than that, I'm not finding much to differentiate them from triceratops in game stats. Perhaps we could give them greater natural armor and/or a chance to avoid critical hits due to the larger "shield"?