The simplest low-cost way is to pick one cloud that runs on all three devices and let it auto-upload everything. For most people that is Google Photos, unless you already pay for Amazon Prime or Microsoft 365. Prime gives unlimited full resolution photo storage in Amazon Photos, and Microsoft 365 includes 1 TB of OneDrive which also auto-uploads from phones. Pick the one you already pay for and otherwise Google Photos is the simplest.
On iPhone, install the app, sign in to the same account, tap your profile picture, go to Photos settings, Backup, and turn on Back up, then in iOS Settings > Google Photos > Photos choose All Photos so it can upload automatically. On Android, install the app, sign in, turn on Backup the same way, then in Photos settings > Backup > Back up device folders choose the folders you want like Camera, Screenshots, WhatsApp. On Windows, install Google Drive for desktop, open Preferences, under My Computer click Add folder, pick your Pictures folder, and choose Back up to Google Photos so new files go up automatically. If you prefer OneDrive instead, turn on Camera Upload in the OneDrive mobile apps and in Windows Settings > Accounts > Windows backup make sure the Pictures folder is syncing to OneDrive.
Google gives 15 GB free shared across Photos, Drive, and Gmail, and the 100 GB plan is a few dollars a month if you need more. If you have Prime, Amazon Photos is effectively free for photos, with 5 GB included for videos. For extra peace of mind, plug in an external drive every month or two and export a copy of everything from the cloud's website to a dated folder, so you have both a cloud copy and a local copy. That way if a device dies or you delete something by mistake you still have a second place to recover it.
