The direct answer
The retention fundamentals — hook, pacing, sound-off captions — apply across Reels, Shorts, and TikTok equally, but the platforms differ in discovery mechanics and audience expectations: TikTok audiences reward native, less-polished formats; Reels benefits from cross-pollination with an existing Instagram following; Shorts sits inside YouTube's broader recommendation system and rewards series-style consistency.
Native vs repurposed content
Content built natively for a platform's specific audience behavior tends to outperform a single video simply reposted across all three unchanged. A version tuned for each platform's caption style, pacing expectations, and discovery mechanics — even with only minor edits — consistently performs better than a blind repost.
Where to start if you can only pick one
Match the platform to where your existing audience already spends attention, and treat the other two as expansion once a format is proven — testing three platforms at once with limited volume usually produces weaker data on all three than focusing on one first.