I think the best way is to use the splash screen and icon generator for Ionic 3.
This works fine for me :
ICON
- Create your icon icon.png or icon.psd or icon.ai. For me, I created my icon 1024x1024 with png extension
- Save your icon into resources directory (example : your-project/resources/icon.png)
- And just use ionic cordova resources --icon
SPLASH
- Create your splash splash.png or splash.psd or splash.ai. For me, I created my splash 4816x4816 with png extension
- Save your splash into resources directory (example : your-project/resources/splash.png)
- And just use ionic cordova resources --splash
In your config.xml file, you will see code generated like this :
<platform name="ios">
<allow-intent href="itms:*" />
<allow-intent href="itms-apps:*" />
<icon height="57" src="resources/ios/icon/icon.png" width="57" />
<icon height="114" src="resources/ios/icon/icon@2x.png" width="114" />
<icon height="40" src="resources/ios/icon/icon-40.png" width="40" />
<icon height="80" src="resources/ios/icon/icon-40@2x.png" width="80" />
<icon height="120" src="resources/ios/icon/icon-40@3x.png" width="120" />
<icon height="50" src="resources/ios/icon/icon-50.png" width="50" />
<icon height="100" src="resources/ios/icon/icon-50@2x.png" width="100" />
<icon height="60" src="resources/ios/icon/icon-60.png" width="60" />
<icon height="120" src="resources/ios/icon/icon-60@2x.png" width="120" />
<icon height="180" src="resources/ios/icon/icon-60@3x.png" width="180" />
<icon height="72" src="resources/ios/icon/icon-72.png" width="72" />
<icon height="144" src="resources/ios/icon/icon-72@2x.png" width="144" />
<icon height="76" src="resources/ios/icon/icon-76.png" width="76" />
<icon height="152" src="resources/ios/icon/icon-76@2x.png" width="152" />
<icon height="167" src="resources/ios/icon/icon-83.5@2x.png" width="167" />
<icon height="29" src="resources/ios/icon/icon-small.png" width="29" />
<icon height="58" src="resources/ios/icon/icon-small@2x.png" width="58" />
<icon height="87" src="resources/ios/icon/icon-small@3x.png" width="87" />
<splash height="1136" src="resources/ios/splash/Default-568h@2x~iphone.png" width="640" />
<splash height="1334" src="resources/ios/splash/Default-667h.png" width="750" />
<splash height="2208" src="resources/ios/splash/Default-736h.png" width="1242" />
<splash height="1242" src="resources/ios/splash/Default-Landscape-736h.png" width="2208" />
<splash height="1536" src="resources/ios/splash/Default-Landscape@2x~ipad.png" width="2048" />
<splash height="2048" src="resources/ios/splash/Default-Landscape@~ipadpro.png" width="2732" />
<splash height="768" src="resources/ios/splash/Default-Landscape~ipad.png" width="1024" />
<splash height="2048" src="resources/ios/splash/Default-Portrait@2x~ipad.png" width="1536" />
<splash height="2732" src="resources/ios/splash/Default-Portrait@~ipadpro.png" width="2048" />
<splash height="1024" src="resources/ios/splash/Default-Portrait~ipad.png" width="768" />
<splash height="960" src="resources/ios/splash/Default@2x~iphone.png" width="640" />
<splash height="480" src="resources/ios/splash/Default~iphone.png" width="320" />
</platform>
If this doesn't work, you probably need to generate the platforms before.