<form class="mainform">
<input placeholder="Имя*">
<input placeholder="Email*">
<br class="clear"/>
<textarea placeholder="Ваше сообщение*"></textarea>
</form>
*{box-sizing:border-box;}
.mainform input{width:49%; float:left; margin:1%; padding:5px 10px;}
.mainform textarea{width:100%; padding:5px 10px; margin:1%;}
}
<form class="form">
<input class="input" placeholder="Name"></inpu>
<input class="input" placeholder="Email"></inpu>
<textarea class="textarea" placeholder="Message" name="" id="" cols="30" rows="10"></textarea>
</div>
.form {
display: flex;
flex-flow: row wrap;
justify-content: space-between;
padding: 10px;
background-color: #f6f6f6;
box-sizing: border-box;
}
.input {
display: inline-block;
width: calc(50% - 26px);
margin: 10px;
border: 2px solid #f24f1d;
}
.textarea {
flex: 1 1 100%;
margin: 10px;
}